The Doctor makes his way through the bustling marketplace on Valhalla, capital city of Callisto, Jupiter’s premier moon. He writes out a notice, advertising himself for sale: one Doctor, in excellent condition, six previous owners and only nine hundred years on the clock, with extensive experience in cross-cultural affairs and science. At the last moment he stops himself writing that he’s wanted in all major star systems and instead describes himself as “much sought after”. He tries to attract the interest of passing traders, but has no success and realises he’s going to have to try a different approach.

Beneath the city, the members of a cable maintenance crew are making their way separately through different sections of the tunnel system. Jevvan receives a communication from her colleague Gerium, who asks if she’s seen Lozz recently as he’s supposed to be working in Duct 7D. She tells Gerium that he didn’t turn up, so Registry assigned her to 7D instead and she found the area was in a right mess! She’s particularly grumpy as it’s her birthday today and she hasn’t even received a message from home because the lines to Earth are still down. Through her built-in camera, she shows Gerium an image of the cabling and he tells her he heard Tin-Marie blaming termites for the damage and that she swore they’d even raided her capsule earlier, moving her fridge an entire foot to the left. At that moment, their colleague Tin-Marie cuts in from Duct 8B and tells him he’s exaggerating - the fridge was only moved six inches. Jevvan says someone has sheared right through the main feed cable, so it’s unlikely to be termites, but when she went to re-route it she discovered someone had been there before. A new feed has already been added, although it’s a real botch job, and it’s coming up through the dead conduits under the old gas refinery. The three of them are angry they weren’t informed about the work being done in their areas and Gerium blames it on Registry cut-backs. He asks if they have their tickets for the monthly riot due to take place later tonight, and Tin-Marie says she has one for the front line. She suggests they file a report and let the Registry sort it all out, but when Jevvan tries to send it, all she gets back is an automated message telling her “every column is many and one”.

When the Doctor arrives at the Job Centre, the clerk invites him to pull up a bean bag. He starts taking down details, but unfortunately the limited information the Doctor is willing to provide doesn’t help much and he still needs to know more about the sort of job the Doctor is looking for. The clerk explains that no one comes to Valhalla City any more, at least not to stay. He describes it as a lousy, vermin-infested bubble in a frozen crater on a moon at the rough end of the solar system. When the gas mines ran dry, Earth declared the colony independent, effectively washing their hands of it, and these days they only get visited by tourists on the wrong flight or losers who can’t afford to get away again. He suggests the Doctor tries Ganymede instead. The Doctor notices the clerk has a barcode on his tongue and he wonders whether it’s a fashion statement or whether he’s been catalogued, but the clerk is surprised the Doctor wasn’t stamped himself on the way in.

As the clerk questions the Doctor further, their entire conversation is being monitored by Mayor Laxton. This is the third employment window the Doctor has visited today, so Laxton checks to make sure that the Registry is referencing it. A female voice reassures him that all systems have been applied and confirms that the subject has not been recognised. Laxton wants to know how he got through immigration without being stamped and he wonders if the Doctor might be from Earth authorities, come to check up on them. With the riot tonight, not to mention Laxton’s pre-riot statement, trouble is the last thing they need. He considers calling the riot off altogether, but then he realises that would be an over-reaction and people have to let off steam somehow. However, when he reads back the speech that’s been prepared for him, he’s surprised to discover it contains references to restructuring. To his horror, he realises Registry has been planning to take over all the time. He demands to know why he wasn’t consulted first, but Registry ignores his protests and insists that he reads the speech that’s been given to him. It is their future.

The Doctor tells the clerk he’s thinking of opening up a hotel and already has a well-appointed property with more than enough rooms. When the clerk points out that he’ll need to employ and train staff, including cooks and chambermaids, the Doctor admits his idea was a little fanciful. He says he’s been finding it hard to settle down anywhere and seems to be endlessly busy, doing hundreds of things at once. The clerk suspects the Doctor needs a psychiatrist rather than an employment allocations operative and he again recommends Ganymede. The Doctor refuses the suggestion and begins handing over all his personal effects, insisting that he doesn‘t want them anymore and that it‘s time someone else took responsibility. The clerk calls out for security, but before they can arrive, a voice comes over the loudspeaker telling everyone to stand by for an announcement from the Mayor. The clerk dismisses the announcement as the usual pep-talk from their leader before tonight’s riot.

Mayor Laxton’s voice booms around the entire city, addressing the citizens of Valhalla. He acknowledges that times have been difficult recently, but he says the cutbacks have been working well. So well, in fact, that they can afford to take more drastic measures. There will be more power cuts and less food, but he’s confident they can withstand the coming storm together. Across the city, people shout out in protest. Down in the tunnels, Tin-Marie and Gerium are more determined than ever to attend that night’s riot and take the opportunity to smash the oppressive Registry. Laxton himself has predicted their reaction and knows they want to riot, but he urges them to follow his advice to stay at home tonight where they‘ll be safer. The citizens refuse to listen, but the Doctor realises the Mayor is trying to warn them. Laxton makes a reference to the other off-world republics in easy reach of this world, and the Doctor is certain he’s telling people to leave. Suddenly, there’s a loss of power and the Mayor’s voice cuts out. The clerk tells the Doctor these power cuts are quite common as the whole sub-city grid is on the slide, but the breakdowns never last long. As the Doctor gives the clerk a supply of everlasting matches, he notices the office has an infestation of termites. The clerk says they’ve been coming up all over the place from underneath the city and the Doctor realises this is where the sub-city grid is located. He admires the termites for being enterprising and says this is evolution in action, then he leaves, telling the clerk he has an appointment.

In the tunnels, Gerium starts to panic when the lights go out, but Jevvan reminds him his vid-com is still running, so he can turn up the brightness on the screen. This is the fourth power spike in the space of a week, so the team start running diagnostics on the junctions. Tin-Marie agrees with Gerium’s theory that Registry is running scared and is trying to sabotage tonight’s riot, but Jevvan doesn’t think this makes sense as it’s Registry that organises the riots. The results of the diagnostic shows a major blow-out on junction eight, which falls within Tin-Marie’s area. Gerium laughs at her misfortune and offers to meet her later at the café if she finishes on time, but then Tin-Marie hears a strange sound getting closer and reports that the pipes are starting to shake. Before Jevvan can persuade her to leave, there’s a loud crash and the communication with Tin-Marie cuts off. Jevvan calls for her friend, but there’s no response. The voice of Registry comes over the vid-com and tells her the signal has been lost.

Power is restored when the local generators come back on line. Mayor Laxton accuses Registry of pulling the plug on him, but the voice assures him it was the result of a power surge. When the voice accuses him of breaking the terms of his contract, he argues that they hadn’t agreed anything about restructuring or a take-over. Registry insists that Valhalla is no longer viable and Laxton accepts that people are fractious because they have no credit and they only just break even on energy, but when Registry points out there are other resources they can use, he reminds her that the contract guarantees no interference. He insists that tonight’s riot be cancelled and any negotiations should go through him rather than an antiquated public service database. Suddenly, the doors close of their own accord, locking Laxton inside. He demands to be let out, but Registry tells him the decisions are no longer in his hands.

Jevvan arrives in Duct 8B and nervously calls out for Tin-Marie. She jumps with shock when the Doctor emerges from the shadows and tells her not to go further into the tunnels. She says no one is allowed down here and he’s not surprised - it’s the home of creatures that scuttle in the dark. He asks her about the power cables and she tells him they feed into the old refinery deep underground, but she doesn’t know who runs it. He wants to go up to ground level, but she insists on staying to look for her missing friend. They’re interrupted by a deep rumbling noise from within the shadows. The Doctor warns her it would be better not to investigate, but she runs off, calling for Tin-Marie. Then he hears Jevvan start to cry, and although he dismisses her behaviour as self-indulgent and seems determined not to become involved, he eventually relents and goes after her…

Crowds begin to form on the streets of Valhalla City and before long, a party atmosphere is spreading through the population. The voice of Registry asks everyone to take their places as this evening’s riot is due to start in 15 minutes. Primary Area 1 is for ticket holders only and anyone without a ticket is forbidden to wander into the cordoned-off zones. The use of weapons above Silo C is also prohibited and body armour and refreshments can only be purchased from licensed outlets. Finally, Registry reminds the crowd that if there’s another power failure, the riot maybe subject to delay or postponement.

The Doctor finds Jevvan weeping in the dark and gently helps her to her feet. They’re standing on the edge of a gigantic hole that’s been dug into the ground and the walls are covered with blood and body parts. She believes the Doctor is responsible and accuses him of killing her best friend, but he examines the hole more closely and realises something has dug its way up into the tunnel from below and then up onto the roof, tearing the metal walls apart like paper and eating solid rock at high speed. He wonders if it’ll come back and decides they should leave. The Doctor finds a vid-com which suggests the victim wasn’t Tin-Marie, but their friend Lozz. They hear the deep rumbling noise again and the Doctor thinks it’s coming from something in pain. Jevvan spots something moving in the shadows down in the hole, but when they start backing away, the floor suddenly crumbles away beneath them. The Doctor manages to grab Jevvan and slowly pulls her to safety. She picks up a pile of thermic charges (standard issue in case the mice down here turn nasty) and throws them into the darkness. Seconds later, there’s the rumble of an explosion. As the dust settles, the Doctor and Jevvan decide not to wait to find out what was making the noise.

The handover procedure commences and Mayor Laxton’s codes are passed to Registry. Resigned to what’s happening, Laxton plans to leave with a degree of dignity intact before the new management takes over and he amuses himself to think about how they’ll cope with tonight’s riot. He asks for his flyer to be put on standby and for his car to pick him up and take him to the spacedrome.

Registry addresses the waiting crowd again through the loudspeaker system, telling everyone to take their places as the riot is due to start in four minutes. Tin-Marie bumps into Gerium and complains about the noise down on level eight. She tells him the area was shaking as though the Mars Express was coming through, so she made a run for it. Gerium urges her to hurry to their places before the riot starts.

The army of termites continues marching, then they come to a stop and start singing. The Doctor recognises it as a song of triumph, so he tries to lead Jevvan in the direction of the spacedrome, but she tells him that’s in the Russian quarter and the direct route is blocked by the termites, coming out of the ducting. Jevvan starts to mourn her friend Lozz, but the Doctor brings her to her senses. The moon has been invaded and the streets are empty because everyone has either been captured or eaten. The realisation, combined with the increasing noise from the termites, overwhelms Jevvan. She decides to head for the café where she’d arranged to meet her friends earlier, so the Doctor agrees to go with her.

Mayor Laxton is becoming impatient and demands to know why his car is late, but Registry won’t release him until he gives them access to all his personal codes. Codes 16-20 are his ticket out of here and he refuses to hand them over until he’s safe on his flyer and without them Registry cannot take control of Valhalla City. Laxton and Registry saved this city when the council abandoned it, so he believes they should continue working together rather than against each other. He’s angry that Registry went behind his back and talked to the termites so he refuses to hand over the ‘keys’ until he’s safely on his way. Registry complies and opens the door - and Laxton find himself face-to-face with the giant termites waiting outside. Determined to fight his way out, Laxton opens fire on the insects…

The Doctor and Jevvan arrive at the Café of Good Hope, the name of which the Doctor finds promising. Unfortunately the place is empty and Jevvan’s friends are nowhere to be seen. They drag a table over to barricade the entrance, even though they know it won’t keep those creatures out. The Doctor decides to take stock of the situation - the café runs on local gas and it still has light and food, so he decides to prepare a meal for them. Chips twice and two teas. Jevvan wonders where the termites came from as she’s certain they didn’t originate here. She hears Gerium calling her name and it takes her a moment or two to realise his voice isn’t coming from the communicator but from inside the café itself, where he’s been hiding all this time! He tells her the city is crawling with termites and there are plenty of people who didn’t survive. He asks Jevvan about the Doctor and she tells him she thinks he’s an investigator come to do a time and motion study.

The Doctor manages to use Lozz’s vid-com to make contact with Registry and he asks to speak directly to Mayor Laxton. The female voice tells him the Mayor is unavailable and asks the Doctor to verify his identity code, but he knows Registry has been watching him all day. When his identity is finally revealed, the Doctor realises he’s speaking to the voice of the termite controller. The Doctor admits that he came here to investigate the unscrupulous business venture they are proposing and, if necessary, shut it down. He also knows that the owner of the voice is giving birth - on a permanent basis. The voice offers to send an escort to pick him up, but the Doctor refuses to co-operate and switches the vid-com off. By the time the Doctor returns to the chips, he finds they’re burning…

Gerium tells Jevvan the riot should have been the best one ever, but then the creatures arrived in their hundreds and people started screaming and running. He asks Jevvan if she trusts the Doctor, but she’s not sure yet. Then the Doctor returns and tells them the chips are off, so they’ll have to make do with biscuits instead. Jevvan introduces Gerium and the Doctor notices straight away that the man’s pockets are bulging with looted items. Tin-Marie contacts them and tells her friends to get straight to the spacedrome as everyone’s being evacuated from Valhalla. Suddenly a worker termite appears outside the window and although it’s searching the area, it doesn’t appear to have seen them yet. Eventually it passes by and everyone relaxes - but then Lozz’s vid-com, which the Doctor used earlier to call Registry, activates. The ringtone blares out and moments later, the worker termite returns to the café and starts scratching on the door, trying to get in. They realise it won’t hold the giant creature back for long, so the three of them race to the back of the building, just as the glass explodes behind them…

Outside the café, the Doctor urges his new friends to hurry as the termite is summoning others. He wants to go straight to the spacedrome, but Gerium insists on returning home first to pick up some of his belongings. Before they can argue, more of the insects appear, so they are forced to flee for their lives. Unfortunately they find the street ahead is blocked by a termite work party which is tearing the buildings apart. Incredibly, they seem to be actually eating the masonry. The Doctor explains that the insects are processing it in order to feed their Queen and although normal termites only digest organic matter, these creatures are ravenously omnivorous, which suggests they’re not natural. Jevvan says they’ll never reach the spacedrome on foot but Gerium spots a huge dumper truck nearby and believes they can get there in minutes. However, there are termites between them and the truck…

The citizens of Valhalla are all gathered at the spacedrome and Registry orders everyone to remain in the departure lounge and wait to be called. A guard tells Tin-Marie they’d better hurry as Valhalla is being written off. Registry announces that the flight to Ganymede is ready to board, but when the guard says there won’t be another flight, the crowd surges forward…

With the Doctor and Jevvan shouting instructions to him, Gerium steers the dumper truck and manoeuvres it at high speed around all the road blocks and temporary barriers that have recently been erected. Gerium demands to know how the Doctor got hold of Lozz’s vid-com, but Jevvan tells him they found it abandoned in the ducting. They approach further road blocks and swerve around them, and it becomes clear that someone is trying to stop them getting to the spacedrome. Every time they head in the right direction, they’re turned back towards Registry. Jevvan doesn’t see how this could be possible as Registry is only a computer and the surveillance cameras are all down. The Doctor notices she also has a bar code tattooed on her tongue and although she says it’s an immigration symbol, he knows they’re also useful for speeding things up at the checkout. Gerium points out that people aren‘t for sale, but the Doctor isn’t sure about that… Suddenly, a group of huge termites appears in front of them and they screech to a halt and back the truck up until they reach the nearest turning. They come face-to-face with another road block, but this time the barrier lifts up to let them through - as if it knew they were coming. The Doctor realises they’re being tracked by Lozz’s vid-com even though he left it behind at the café and it soon becomes clear that Gerium had stolen it. The vid-com rings and Gerium answers it. The truck races into a dead end and Registry explains that she’s sent an escort for the Doctor. The termites start to crawl towards the truck, but the Doctor throws the vid-com away and orders Gerium to drive away quickly. Gerium realises it’s the Doctor the invaders are after and considers throwing him out of the truck, but before he can act, the air is filled with a deep booming growl. The Doctor warns Gerium to drive for his life or they’ll never get off this rock alive!

At the spacedrome, Registry orders everyone to stand back as the flight to Ganymede is now at full capacity and no more passengers can be accommodated. Tin-Marie calls Jevvan on her vid-com and tells her there’s still a crowd of people stuck outside. As Registry advises people to await further announcements, Tin-Marie wonders whether any requests for help have been sent to Ganymede or Earth. The Doctor, Jevvan and Gerium are now outside the spacedrome, but there’s a lockout and they can’t get the gates to open. As a level three electrician, Gerium tries to bypass the circuit, but it’s on a separate grid to the Registry ones he’s used to. Unfortunately the termites are massing nearby and just when they don’t think things can get any worse, the last flight off the moon lifts-off and disappears into the sky, leaving everyone else trapped behind. Jevvan is devastated - she used to come here on her days off and watch the freighters unloading from the viewing gallery. Now, they’re all stranded here and no one else ever stops on this moon, not even the tankers passing through the space lanes on their way to Ganymede. The Doctor orders them both to stop wallowing and starts work himself on the gate. Although he takes 430 volts in an electric shock, it doesn’t have any effect on the gate. They hear the deep growling again and watch in horror as the termite army starts moving towards them. Then they notice something else moving ahead of the termites, getting closer every second. It’s Mayor Laxton’s limousine, heading straight for the gates. The metal doors open up automatically, letting the car through and allowing the Doctor’s party to gain access too.

The guard orders the gates to be sealed immediately and then approaches Mayor Laxton and guides him towards his personal flyer. The crowds are forced back to allow the Mayor access and as they clear a path, Jevvan and Gerium notice the Doctor has disappeared. Suddenly Tin-Marie’s voice booms out of the loudspeaker system, demanding that Laxton tells them when help will arrive and where the emergency teams have got to. The crowd begins to turn hostile so the Mayor addresses them and assures them help is on the way. Tin-Marie points out there are dead and injured who need help now, but Laxton says the links to Ganymede and Earth are down, so he’s going to get assistance personally. Tin-Marie believes they’re being abandoned, but before she can say anything more, she’s grabbed by some of Laxton’s supporters and pulled away from the microphone.

Laxton is led towards his ship, but as he enters the main building he’s approached by the Doctor. The guards draw their weapons, but Laxton calls them off. Out of earshot, the Mayor asks the Doctor if he’s there to investigate him, but when the Doctor says he’s already spoken to Laxton’s employer, the Mayor realises the Doctor has never seen Our Mother the Fourth, the termite Queen and progenitor of all her race. This is her city now and she has plans for Valhalla. Laxton wonders whether the Doctor has been offered his job, but the Doctor reassures him the position of Mayor isn’t really his style…although she might be interested in getting a marketing consultant. The Doctor knows the termites aren’t natural and asks Laxton if they were created by the humans, but the Mayor refuses to answer any more questions and is led away.

The Doctor protests at receiving the summons and insists that he has an investigation into Our Mother’s business practices to complete. He agrees to talk to her, but only on equal terms, and asks if she’s considered the damage she will cause - not to the humans, but to her own people. He insists that she releases all the humans and let them leave, and Jevvan, who has been watching from a distance, realises he must be bargaining for their lives. The Doctor offers to find the termites something else to sell if they agree to abandon their plans now, but the Queen refuses to let him destroy her children’s future. She asks to speak to Laxton, and although the Doctor says his ship left ages ago, she knows this is not true. The termites start moving in the direction of the Mayor’s flyer and the Doctor realises Laxton is going to be in for a bumpy take-off!

The crew aboard the Mayor’s ship take up their positions in readiness for the launch. Clearance for departure is received and Laxton settles down for a drink. Suddenly an alarm sounds and the pilot contacts Laxton to tell him the termites are on the hull. Outside, Jevvan leads the others to the viewing gallery which overlooks the dock. They look down and see that the termites are crawling all over the flyer and the Doctor realises they must be after Laxton himself as he’s the liaison between their world and the humans. The insects start cutting the ship out of the cradles and then incredibly they turn the entire ship upside down and carry it away on their backs. The Doctor reveals that Laxton knew the invasion was coming and although he tried to warn the people, his dash for freedom has been well and truly scuppered. The Doctor borrows Jevvan’s vid-com, but the line to Registry is unavailable. Now that Our Mother has Laxton back, it seems the Doctor is no longer needed.

Laxton and the crewmen aboard the ship struggle to hang on as their environment is turned on its head and they find themselves strapped to the ceiling. Our Mother mocks him for his failed escape attempt, but he argues he was only trying to save the people of Valhalla. She taunts him and says humanity’s time at the top of the stack is now over. She still needs him to release the final codes to the Registry, but he demands that she release the people first. Our Mother realises she undervalued him and tries to find out more about the Doctor, and Laxton suspects that she’s scared of him. He doesn’t recognise the area his flyer is being carried down and it slowly dawns on him that the termites are ripping up the entire city so that the streets and buildings are no longer recognisable.

The Doctor tries to gain entry into the spacedrome freighter dock, despite Jevvan’s assurances that they’re no longer being used and will be empty. He manages to break the entry code and the doors open, allowing him, Jevvan and Gerium to gain entry, whereupon he proudly shows them his own ship, the TARDIS. Gerium touches the box and is amazed to see that it carries a slight energy pulse. Meanwhile, Jevvan has been observing events down in the city and realises the termites are actually dragging the buildings across the landscape. Even Tchaikovsky Tower, the block where Jevvan and Gerium live, is on the march, being moved through the city like a chess piece being played by a giant. The Doctor advises them to come away from the window, but it’s too late. They see one of the termites trying to enter the freighter dock from below, physically eating its way through titan steel. Gerium starts to panic and demands that the Doctor open the bay door again so they can escape, but instead, the Doctor orders them inside the TARDIS. He hangs behind for a second and approaches the creature, addressing it politely and asking if it understands him. It’s a heavily armoured soldier termite and the Doctor guesses correctly that it’s in direct communication with its co-workers and Our Mother herself is listening to everything he says. She wishes to be rid of him once and for all, and on her instruction, the soldier termite increases its efforts and starts breaking through the bay door. Jevvan has seen enough and borrows some thermic charges from Gerium and hurls them towards the advancing creature. The Doctor is furious as she’s only succeeded in making things worse. More and more of the insects arrive outside and charge towards them. The Doctor grabs Jevvan and Gerium and literally forces them inside the TARDIS.

The two maintenance workers are amazed at the sight that greets them inside the ship. Gerium starts to panic again, but the Doctor assures them they’ll be safer here than they were outside. He compares his TARDIS to the human mind - fairly small in appearance, but fully functional and chock full of ideas. Jevvan assumes he’s planning to use the ship to help get the people off Callisto, but the Doctor seems to have other ideas. Gerium offers to pay for his personal salvation, but the Doctor refuses to become a taxi service, despite the constant stream of non paying guests, freeloaders and sightseers he seems to pick up. Neither he nor the TARDIS have any intention of going anywhere at the moment - or so he thinks until the army of termites outside physically lift up the ship and start carrying it away!

Our Mother continues to put pressure on Laxton to open the doors to his flyer, but he refuses to co-operate and claims to have wiped the remaining Registry codes. Moments later, the worker termites break through the hull of the ship and swarm inside. Our Mother tells him that in order to set up her business, the ship’s computers must link to the codes he hid from her. Despite his efforts to stop her, the connection is made and the access codes are finally released. The Queen orders the resource registration to commence immediately and it begins by calling up a list of the crew. As each name is read out, Laxton can hear screams coming from deep within the ship. He begs them to leave the crew alone and even offers to surrender in their place, but his pleas are ignored. The termites swarm over him and within seconds he is completely cocooned and unable to move or speak. Our Mother announces that she is approaching the surface of the moon and orders new egg chambers and new quarters for her grooms and nurses to be arranged. Soon her race will throw open their gates for business!

Inside the TARDIS, the Doctor urges Jevvan and Gerium to hold on tight as the three of them get thrown around the control room. Jevvan finds herself hanging above a side door which effectively opens up to a corridor the length of a mine shaft. The Doctor tries to operate the controls on the console, but by now it’s on the wall and beyond his reach. Gerium slips and Jevvan grabs hold of him as he falls past her. Eventually, the Doctor is able to operate the pitch stabiliser control that allows the inside of the ship to turn upright. As everyone recovers, the Doctor activates the scanner and is intrigued by the nature of the insects, wondering if they’re mutations or freaks from some blind evolutionary alley. Gerium refuses to get involved, but the Doctor tells him it’s too late and produces a full readout of Gerium’s confidential credit ratings and details of his previous convictions for petty theft. The information is coming from a sales catalogue programme which the Doctor found two hundred years in the future in an old curiosity shop on another planet. The catalogue lists everyone in Valhalla and shows that they’re up for sale!

Tin-Marie Fisher accesses Registry in order to send a mayday message, but her requests don’t seem to be getting her anywhere and she‘s not surprised when the computer tells her all comms are closed. The computer assures her an emergency signal has already been sent, then it puts her on hold. She’s forced to endure a promotional trailer for a new service being offered from Callisto, inviting her to register for an exciting new human resource trade-off opportunity. When she gives her name to the computer again, it seems to misunderstand her request and informs her that this purchase is not yet available. Suddenly the signal is interrupted by a message from the patrol ship Resolute, which has picked up the mayday signal and is coming in to land to offer assistance.

Inside the TARDIS, Gerium checks the catalogue and finds Jevvan’s name is listed too. The Doctor confirms their worst fears - the catalogue is a glossy hi-tech slave market showcasing the citizens of Valhalla from lowly waiter up to executive Mayor. Jevvan is amazed that there’s still a market for slaves these days, but the Doctor says the galaxy is a cosmopolitan place and there’s no limit to what can be bought or sold, either as slaves or as food, or even as laboratory specimens. Gerium notices the brochure is dated 9146, which is just one month away. The Doctor prepares to negotiate with Our Mother, but he’s determined it will be on his own terms. If he can make their Queen see sense, he can steer her onto a new course for a new and better future. Jevvan realises that if the Doctor comes from the future, he must know what’s going to happen to them, but he changes the subject and tells her he still has some enquiries to make. In fact, the Doctor himself is also listed in the catalogue and he knows there are people across eternity who’d pay a fortune for his head on a silver platter. Jevvan insists they do whatever they can to rescue the people at the spacedrome, but then she realises the Doctor has gone…

The patrol ship Resolute is on its final approach and the captain contacts Tin-Marie again to confirm details of their situation. She explains that the city has been invaded and they require emergency evacuation, but when the captain repeats his request, Tin-Marie realises he’s unable to hear her. Instead, the signal is being interrupted by the voice of Registry who speaks directly to the captain. Registry tells him they have meteorite damage to one of their substations and any help he can offer would be appreciated. Tin-Marie tries to cut in, but is unsuccessful. She listens in horror as Registry clears one of the space docks in readiness for the arrival of the Resolute, then welcomes the crew to Valhalla…

Jevvan and Gerium are horrified to discover that one of the termites has entered the TARDIS. Fortunately it seems to be more interested in the control console, so Jevvan tells Gerium to make for the door while she tries to distract it with a handy umbrella she’s found. She strikes the termite and in a flash the creature changes its appearance back into the Doctor. He explains to the totally perplexed maintenance crew that he didn’t actually change shape, but the TARDIS’s telepathic circuits altered their perception of him so they thought they saw something different. It was nothing more than an illusion, a sleight of mind. Gerium is furious and even Jevvan asks him never to do that again, but he plans to play the same trick on the termites and hopefully they’ll mistake him for one of their own kind and then he can get to the Queen. Just at that moment, the TARDIS comes to a stop and the termites outside seem unduly excited about something. The Doctor sets the TARDIS to listen to the termites in the hope that the ship can understand the meaning and structure of what the creatures are saying and translate for him. The control room is filled with the babble of meaningless overlapping thoughts - the entire telepathic net of the termite world - which the Doctor then narrows down to specific individuals. To his joy, he hears the leader of the soldier termites issuing military orders to his troops. He switches the ’disguise’ back on, which turns him back into the image of a termite, and programmes it to give him the rank of a General. Now armed with the ability to translate their language as well as take on their appearance, he is ready to go outside and meet the Queen. As he leaves the TARDIS, he gives orders for his human friends to close the doors behind him…

The termite immediately outside the TARDIS plans to “bite the plunder-box open”, but his colleague warns him that Our Mother has ordered that the contents are not to be harmed. The workers unite in a chant of praise for their Queen’s forthcoming ascension when the TARDIS door opens and the Doctor emerges, demanding to know what’s going on. The first termite asks him what he was doing inside the box but the Doctor claims to be a superior officer and demands to know who’s in charge. The termite calls for the Sergeant and the Doctor tells him he was inside the plunder-box on the orders of Our Mother the Fourth to guard the valuable contents. The Sergeant challenges him and asks him to give today’s watch words, leaving the Doctor no option but to bluster his way through.

The entire conversation is being observed by Jevvan and Gerium inside the TARDIS. They realise the Doctor is floundering, but Jevvan remembers the messages Registry had been sending out over the vid-coms. Although the latest thought is “today is the first step in the march to the stars”, the Doctor appears to be throwing out random guesses that are nowhere close to the correct watch words. Jevvan tells Gerium she’s going outside to help the Doctor. She was watching how the Doctor set up the camouflage programme and is confident she can duplicate it. She adjusts some of the controls and before Gerium‘s eyes, she’s instantly transformed into the image of another termite. Surprisingly, Gerium himself has also changed.

The Doctor, still adopting the guise of a termite General, addresses Our Mother and tells her the Doctor is an elusive fellow and hard to catalogue. She believes him to be acting as the Doctor’s envoy and summons him up onto her wagon, telling him she values his counsel. He discreetly whispers to Jevvan and Gerium and tells them to make their way to the spacedrome and make as much disruption as they possibly can in order to delay things. He climbs aboard the chariot and sits beside Our Mother, then the Sergeant calls the workers to action and the entire procession marches forward. The soldier notices Jevvan and Gerium are loitering behind so he orders them to attend to the hatchlings’ dinner.

The Doctor tells Our Mother that the reconstruction of the city is proceeding apace. The old buildings are on the move and are being cannibalised to fortify the Registry as their new termitary. Our Mother tells him she can remember every single one of her children - every worker and every soldier - but she doesn’t know him. She orders him to move closer, and although she finds it hard to focus on him at first, she soon realises that he is the Doctor. Before she can decide on any action, a nurse approaches with news that her current egg-rate has dropped to just two a minute. Our Mother is shocked to discover how un-reproductive she’s become, but the Doctor suggests her workload may be too heavy and recommends that she abdicate and undergo a full medical check-up. Instead, she summons her heroic workers closer and proudly tells them how well their new city is coming along. She believes this will lead them to ultimate glory, and the army of termites cheer in celebration. She warns the Doctor that she could crush him at any time and he accepts defeat. He realises there’s no point in hiding any longer and removes the chameleon disguise, standing before her in his full glory and challenging her to do her worst. He promises that all he wants to do is help her and stop her from leading her people into absolute disaster.

Assigned to work duty attending over the eggs, Jevvan and Gerium are thrown around as the army of termites step up their pace and triple march towards the Registry. As they’re forced to speed up, they realise they have no hope of getting away now.

Our Mother emits a long drawn out moan and asks the Doctor if the humans have sent him to plague her. He offers to begin an examination of her and reports that her blood pressure is very high and her reflexes are almost negligible. He asks her how she thinks the humans will react to the news that she’s turning Valhalla into a retail park, but she reminds the Doctor that Earth turned its back on this colony long ago. In any case, she believes the termites to be superior, industrious and intelligent. She tells him that long ago in the reign of Our Mother the First, humans genetically engineered a team of heroic termite pioneers to burrow into the heart of Callisto and seek out new energy sources. When Earth cut Valhalla off, no one even remembered them, but they kept working, sending years worth of data back to the Registry, and before long the traffic of data started going both ways. Their huge size is explained by the fact that they worked outside Valhalla’s gravity pan, and Jupiter’s infamous gravity fluxes caused them to grow massively in just a few generations. The Doctor warns her that humans are difficult to keep as captives as they go off their food and lose their market value. He then breaks the news that her condition is in terminal decline due to bad diet and constant stress, but she surprises him by revealing that she already knew she doesn’t have long to live. He urges her to stop her disastrous plan now for her people’s sake, but she still believes it will be their moment of triumph. She orders the registration to begin…

Both Jevvan and Gerium have noticed that the eggs they’re watching over are starting to shift, as though they’re ready to hatch. They seize the moment and edge towards a nearby street. When they’re spotted by the Sergeant, they both lift their eggs and threaten to smash them, but before they can act, the eggs crack open and huge baby termites emerge. The two human drop the creatures and run for their lives, with the worker termites chasing after them…

At the spacedrome, Tin-Marie takes control of the loudspeaker system again. She addresses the crowd and tells them security has sealed off the buildings because the termites have broken in. They face a simple choice - they can either sit there, or they can fight back. She gathers together a small group to hack their way through to the communications system in the hope they can get a signal out while the others keep the termites busy. They only have one chance and they’ll all have to work together to create a distraction. She suggests they hold their own riot, one that doesn’t need a ticket to take part. The crowd cheers in support and they begin to put their plan into action.

A worker termite warns Our Mother that the human livestock at the spacedrome is growing agitated. She insists that full registration be instigated and orders her workers to carry her inside the Registry. Unfortunately the entrance is too small for her to pass through, so first they must widen it by eating away at the structure. She can feel her innards churning and the Doctor advises her not to move. He suggests he be allowed to enter Registry himself to retrieve the necessary drugs and reluctantly she agrees, but she orders a worker termite to watch him carefully and if he tries any tricks, it should slice him to pieces!

The streets are crawling with termites, so Jevvan and Gerium decide to lose their disguises as they feel they stand a better chance of getting to the spaceport without them. They’re surprised to see they’re not far from the Tchaikovsky Tower, which has been moved, lock, stock and barrel.

Inside the Registry, the final access codes start downloading. The Doctor learns that the registration process transfers details of the human livestock onto the catalogue, like a form of stock taking. He notices a huge egg nearby and wonders why it hasn’t been taken to join the others in the hatchery. He uses a tuning fork to crack it open and the coughing, spluttering form of Mayor Laxton emerges. The Sergeant sounds an alarm, alerting the others to the fact that one of the livestock has escaped, then orders the Doctor to stand away. Before anyone can arrive to help, Laxton shouts an override instruction to the computer and a blast-shield barrier immediately drops, crushing the Sergeant beneath it. Laxton knows the barrier won’t keep the termites out for long so the Doctor quickly tells him about the Queen’s plans. He asks Laxton to unlock the comms system and use his broadcasting skills before Our Mother drags everyone into the abyss.

Tin-Marie leads the crowds into a mass protest and forces open the door to the spaceport, but is forced to close them immediately when more of the termites start swarming in. The voice of Registry comes over the loudspeaker, ordering people to step forward once their number is announced. The first name to be announced is Richard Harrison Josephs. The man nervously moves forward and is surrounded by termites. Within seconds, he’s completely cocooned within a freshly spun casing. The rest of the crowd is horrified as it slowly dawns on them that they too are destined to be packaged inside the shells. The next name is announced and another man reluctantly steps forward to be encased within a cocoon.

The Doctor urges Laxton to hurry. Eventually, the Mayor is ready to broadcast, but before he can proceed, the blast-shield barrier smashes open and the termites swarm in, accompanied by Our Mother. She accuses the Doctor of tricking her, which he openly admits, arguing that her slave trade is offensive to the whole universe and promising to close their operation down. The termites surround them and Our Mother announces that she intends to put the Doctor on the open market to see how he fares…

The spacedrome is now filled with hundreds of eggs. The next slave to be processed is Tin-Marie Fisher, but when Registry orders her to step forward, she refuses to co-operate and has to be forcibly grabbed by the termites. Suddenly, there’s an enormous crash and the dumper truck smashes through the wall, driven by Jevvan and Gerium. When the dust settles, they’re stunned by the realisation that virtually all of the humans have been cocooned - and in their confusion, they’re overcome by the termites.

Our Mother explains with pride that the livestock in her catalogue features a broad range of skills, but the Doctor objects to this and insists that humans are people. Laxton reminds her that she deceived him when she first made contact, claiming her race had discovered the new energy source he was seeking, but the Queen is dismissive of humanity’s idea of civilisation and argues that her species can do much better. The voice of Registry announces that interest is being shown in the Doctor since his details appeared in the catalogue and the number of people making enquiries about him is rising by the second. Suddenly an alarm sounds, warning of an attack taking place on the registration process and Our Mother orders her soldier termites to return to the spacedrome to protect the livestock. During the distraction, the Doctor orders Laxton to make his broadcast quickly, but as the Mayor races over to the communicator, he is grabbed by the Queen, who starts crushing the life out of him. The Doctor has no choice but to send the message himself…

The Doctor’s voice booms out from the loudspeaker, announcing, with the deepest regret, the passing of Our Mother the Fourth, who he says died peacefully in her sleep. Our Mother is horrified and orders her termites to stop the Doctor, but through the window, Laxton can see thousands of the termites already coming out of the city and swarming towards them. This is their wedding flight in which the grooms take to the air in a frenzied hunt to find a new Queen. The Doctor instructs Laxton to flush Registry of Our Mother’s influence and get help for his people. The Mayor approaches the Queen and rebukes her for not only lying to him, but for showing contempt for his position. Ignoring the Doctor’s request to be careful, he moves to break the link to Registry and he pulls out the cable - whereupon he is killed instantly by a massive electric shock. Moments later, the Queen also collapses, finally worn out by her permanent state of pregnancy and the stresses forced upon her body.

Tin-Marie approaches Jevvan, who is crying over the body of Gerium, and the two women are overjoyed to be reunited. Tin-Marie tells her she was packaged up in one of the eggs until Jevvan called her vid-com and woke her up. She kicked her way out of the cocoon and found all the termites had gone, so she went to the patrol ship Resolute and sent off the mayday to Ganymede and now rescue ships should be on their way. The two women are the only humans who haven’t been processed and the Registry now contains literally thousands of eggs. Tin-Marie doesn’t know how they’re going to wake everyone, but the answer seems obvious to Jevvan. Each person has their own vid-com, so Jevvan sends out a blanket call. Seconds later, the room is filled with the sound of ringtones as thousands of vid-coms activate at once. Slowly, one by one, the colonists start to claw their way out of their individual cocoons.

Old Mother is close to death, but she can see the sky is filled with flying termites and realises they must have found a new Queen, Our Mother the Fifth. She wonders whether time will be kinder to her successor, but the Doctor tells her time has a tendency to sneak up behind people and mow them down. He admits that he’s had enough of being himself and was attracted to her catalogue filled with nice, straight-forward, uncomplicated jobs. He even considers advertising the TARDIS, but Our Mother knows he cannot deny his destiny and he should consider himself fortunate to be in such demand. The Doctor realises there’s still much for him to do and the Universe is too busy…but then he discovers the Queen has finally passed away. In the sky above them, the termites continue to swarm…