The Doctor asks Lucie if they know each other. He admits that his long term memory isn’t what it was and the last 300 years or so have been a bit of a blur. He looks at her clothes and guesses Lucie comes from the 1980s, which infuriates her. She tells him they met in the early 21st century when she was dumped on him by the Time Lords on a witness protection scheme. She recounts some of their adventures and then tells him they were the best of mates, but the Doctor is struggling to keep up and thinks it‘s because he hasn‘t heard a Cockney accent for so long. Lucie slaps his face and is delighted when he says it really hurt. Selta tells Lucie she should leave, a suggestion that she readily agrees to. Once they’re alone, Selta wonders whether all Earth females are so aggressive. She turns the Doctor’s attention back to more pressing matters such as the atmospheric scanner and the mystery of Orbis’s moon. The Doctor suddenly realises that Orbis never actually had a moon before and recalls that he first saw it just a few decades ago.
The TARDIS materialises and the Headhunter emerges to be greeted by her contact, Secretary Saccostrea. They’ve never actually met before and the Headhunter rebukes the Molluscari for being late. She then refuses to accompany Saccostrea to meet their leader and insists that Crassostrea be brought to meet her.
The tights work and the Doctor starts to power up the crashed spaceship. Selta wonders if the Doctor is thinking of going back to Earth, but this just makes him laugh. She’s been thinking about what happened with Lucie and suggests the Doctor tries talking to her, but the Doctor has discovered the storms are being caused because the moon is coming ever closer to the planet. This confirms what they learned from the Molluscari data pearl. He’s sure it’s no coincidence and decides to contact the Galactic Council to see if they can persuade the Molluscari to back off. What he really needs to do is visit the moon itself, and Selta suggests asking Lucie if she knows what happened to his TARDIS. The Doctor suddenly realises that in order for Lucie and Selta to have understood each other, the TARDIS must be here somewhere, translating their speech. He races off to find his old companion.
From the safety of the Molluscari ship, the Headhunter and Crassostrea oversee an underwater expedition. The results of the search are negative, so Crassostrea insists that the whelks go deeper, even though Secretary Saccostrea warns that the pressure will cause their shells to crack. The Headhunter and Crassostrea have come to an arrangement - the Molluscari will retrieve something for her from the bottom of the ocean in return for her help persuading the Doctor leaving Orbis forever. The Headhunter has already arranged for the Doctor to be reunited with Lucie and once she gets what she wants, she’ll fulfil her side of the bargain. They listen to the screams over the communicator as the diving shoal finally succumbs to the pressures of the depths, but Crassostrea insists that Saccostrea sends down more of their people…
The Doctor catches up with Lucie and apologises, explaining that he’s had a lot on his mind recently. He asks her where the TARDIS is, and when she tells him the Headhunter has it, he vaguely recalls her as a part-recruitment consultant and part-assassin. The Doctor insists that Lucie slap him again and says that because she’s just come from the TARDIS her fingertips are still charged with chronon particles. When she hit him earlier, it reinvigorated his dormant neurons which is helping him to remember things. There’s still a lot of confusion in his head - he can remember shouting, screaming and doors being slammed - but it’s a promising start. However, Lucie is shocked when he reveals that he won’t be leaving Orbis. He says he’s happy here and he only needs the TARDIS to find out what’s happening on the moon and stop the Molluscari. Lucie insists that he’s a traveller by nature, but he tells her that’s all in his past and Orbis is now his home. Angered by this, Lucie slaps his face several times, but the sight of her apparently attacking the Doctor just causes Selta to race over to stop her. Selta reveals that bloom-father Yanos has received a message from the Galactic Council so the three of them head for town to discover what the news is.
Eventually the Molluscari’s underwater search is successful and the crew pick up the signal that the Headhunter has been searching for. She orders them to trawl the object in and then Crassostrea prepares to confront the Doctor…
Back at town, the Doctor has contacted the Galactic Council and learned that they’re not prepared to hear any appeals until six orbits have passed to allow for a cooling-off period. The Doctor tries to argue that there’s more going on here than they know and says the passage of the Orbis moon is being adversely affected by an external agency. However, the Council refuses to engage in any further discussion and cuts him off. Just then, the sound of an approaching spaceship is heard, signalling the return of the Molluscari. The voice of Crassostrea breaks in on the radio, telling the Keltans their tenancy of the planet is over. She claims Orbis for their own, but adds that in the spirit of inter-species solidarity she will allow a select number of Keltans to be transported to an artificial enclave elsewhere in the galaxy. The selection will be based on weight and breeding potential and she asks for volunteers to assemble on the beach.
The Keltans turn to the Doctor for ideas, but he says he needs more time to think. Just then, the Headhunter arrives and mocks the Doctor for being unable to save the Keltan race from extinction, despite living with them for 600 years. She tells Yanos that the Molluscari’s offer is the best one they’re going to get and advises them to take it. Lucie starts to argue with her, but to everyone’s surprise, Selta grabs hold of her and agrees with the Headhunter, assuring Yanos they have no alternative. Lucie bites her captor and is released, then the Doctor hands Selta his sonic screwdriver for protection and tells her to meet with the Molluscari and keep them talking as long as she can. Once the Keltans have gone, Lucie tries to punch the Headhunter - but the woman pulls out her gun and shoots her again at point blank range.
Saccostrea informs Crassostrea that the Keltans are beginning to gather on the shore as requested. The leader laughs and summons more clam-ships to land on the beach and begin the processing.
The Doctor tries to revive Lucie and is surprised when the Headhunter says she’s been shot with a time bullet. It’s currently burrowing its way into Lucie’s chest at a rate of one millimetre every second, but she can speed it up, slow it down or even remove it entirely as she wishes. The Headhunter tells the Doctor she’s reverse the damage and even return his TARDIS to him if he agrees to leave Orbis. However, he must also agree to switch off a device she’s extracted from the seabed. It’s encrusted in coral after resting at the bottom of the ocean for 600 years, but the Doctor recognises it as the remote activator for the Stellar Manipulator, which he took from Morbius and was holding when he fell into the canyon on Karn all those years ago. It was to protect this device that the Sisterhood transported him here in the first place. The technology can only be activated by a Time Lord, which is why the Headhunter arranged for Lucie to be brought here. She orders the Doctor to switch it off or else she’ll allow Lucie to die. He refuses to co-operate and accuses Lucie and the Headhunter of plotting this together. Until they reveal what their plan is, he intends to keep the activator safe in his pocket. Convinced that the Headhunter won’t let her ally die, he leaves them to their ‘act’ while he attends to those who genuinely need his help. Lucie cries out to him to help, but he isn’t listening…
The Great Crassostrea addresses the Keltans and says that although the five clam-ships they’ve brought to Orbis can’t possibly carry all of them, a fleet fifty times the size will shortly be arriving here as back up. As she orders her troops to round the colony up into manageable groups, she’s delighted to see Yanos turning green, the Keltan colour for fear. She reveals that green Keltans are said to make the best food, so in order to ‘prepare’ the prisoners, she gives her troops permission to terrorise them…
Still believing that Lucie works for the Headhunter, he’s not too surprised when she races out after him. She pleads with him to believe that she really is dying, but it’s only when she inadvertently reveals that the TARDIS is also dying and is belching out time waste over the Vortex that she grabs his attention. The Headhunter joins them and points out that something‘s happening on the beach and it smells suspiciously like a barbecue…
Crassostrea order her troops to be careful not to burn the Keltans with their electro-spears. She tells Yanos the Molluscari need the waters of Orbis in order to spawn, but before they can do that they need to feast. Selta protests, demanding that they release her bloom-father. She produces the Doctor’s sonic screwdriver and activates it. As the Molluscari start to vibrate inside their shells, Selta concentrates its effects on Secretary Saccostrea and within moments the woman is dead. To Yanos’s horror, Selta threatens to do the same to Crassostrea unless she honours their bargain. She activates the sonic screwdriver again - but this time the Doctor steps forward and takes the device from her hand, insisting it was meant to be used for protection, not as a weapon. Selta apologises for her actions, but it’s too late and the Doctor can’t hide his disappointment in her. Crassostrea suggests they don’t waste any sympathy over Saccostrea as she was planning on eating her later anyway.
The Doctor asks Selta to explain what she meant by “honouring their bargain“, and although she avoids the question he already knows the answer. He points out that the readings on the Molluscari data pearl were identical to those shown on the atmospheric scanner, which means they were both taken from the precise same spot on the surface of Orbis. He denounces Selta as a Molluscari agent and she explains that she was forced to help as the Molluscari had predicted Orbis faced a catastrophe that even the Doctor couldn’t save them from and had offered to save a few of her bloom-kin. She herself had already decided to sacrifice herself and stay behind with the Doctor. She pleads with him to forgive her, but he says he only wanted a friend, not someone who was devoted to him. He orders the Molluscari to leave the planet and never return.
The Headhunter arrives and points out how ridiculous the Doctor is for thinking he can hold of an entire alien army with one sonic screwdriver. She gives orders for the TARDIS to be brought out from the Molluscari ship and then Crassostrea tells the Doctor he must go and leave the Keltans to their fate. The Doctor insists that Orbis is his home now and he considers the Keltans to be his people, so he refuses to desert them. Lucie eventually joins them and is shocked to discover the Molluscari plan is to eat the jellyfish-people. Crassostrea has heard enough and orders her troops to kill the Doctor and Lucie, but the Headhunter interrupts, pointing out that the Doctor still has the Stellar Manipulator activator and he hasn’t switched it off yet.
Lucie makes an impassioned plea for the lives of the Keltans and the Doctor seems to come to his senses, finally remembering who she is and realising how much she must have suffered when she thought he was dead. He also takes the opportunity to remind Selta that he’s not really that old, considering how long-lived his race is. He admits that he long ago stopped counting the years and now tends to round his age up or down depending on which part of the Universe he’s visiting. He places the Stellar Manipulator activator in his hand and orders the Headhunter to “un-shoot” Lucie. She agrees and Lucie is returned to normal, but then the Doctor tricks the Headhunter and instead of switching the activator off, he actually increases its power.
At first nothing happens, but then suddenly there’s a huge clap of thunder and then the entire sky goes completely white. The Keltans start to scream in panic, believing it’s the end of their world, and they watch as the entire moon starts crashing towards Orbis. The Headhunter explains that the moon first appeared in tweenspace a few centuries ago when it was tiny. Since then it had been getting bigger and bigger as it was drawn to a spot 50 miles out to sea - where its controller was located. It’s not a moon at all, but the Stellar Manipulator engineered by Morbius! It’s been dormant for centuries until the Headhunter dragged its activator from the seabed and then the Doctor turned up the power. He tries to switch it off, but the controls are jammed and he can’t reverse the effect. As the oceans start boiling, the Doctor orders Crassostrea to evacuate everyone aboard the clam-ships, but it’s too late - thanks to the increase in the ambient temperature, the Molluscari leader is starting to spawn! The rest of the oyster race go into a feeding frenzy and the Headhunter congratulates the Doctor on messing things up so well.
She reveals that the Manipulator was being drawn closer to Orbis and was probably only days away from auto-igniting anyway, so all the Doctor has done is speed things up. She urges them to go back to the TARDIS, but the Doctor refuses to leave the Keltans behind. Unfortunately the effect has now become so great that the Keltans, including Yanos and Selta, start dying all around them before anyone can act. To the Doctor’s surprise, the Headhunter takes the activator and throws it into the sea where it will be destroyed along with the entire planet. Horrified by what’s happening, the Headhunter opens the door to the TARDIS and Lucie urges the Doctor to join them inside - but the Doctor says he gave up the life of a traveller a long time ago and his life here means everything to him. He knows he has to die some day and he seems prepared to accept that today is as good as any. Lucy grabs the gun from the Headhunter and shoots the Doctor with one of the time bullets.
Back in the TARDIS, the Headhunter “un-shoots” the Doctor and he instantly recovers. He’s angry with Lucie, who admits that she was acting selfishly. He tells her there are times when people don’t actually want to be saved and she should have respected his wishes, but she argues that she didn’t just do it for herself, but for Earth and the rest of the Universe. She reminds him that his sick TARDIS is destroying the Vortex, but the Headhunter laughs and admits that she made the whole story of time waste up in order to motivate Lucie. Lucie goes to attack her, but she raises the gun again and warns the girl off. The Doctor is determined to return to Orbis, but when he checks the controls he can’t find the planet anywhere. The Headhunter reveals that Orbis has been consumed by the Stellar Manipulator, but with its activator consumed too, the Manipulator has been destroyed as well. The Headhunter ignores the Doctor’s anger and points out that while he was happily living on Orbis for 600 years, someone else had to take over saving the Universe. She and her associates are getting sick of it and they’ve now decided that the Universe can’t do without the Doctor. The Doctor insists that he’s a different person now, but she says that’s a problem he needs to sort out for himself. She says goodbye and teleports from the TARDIS back to her warp ship.
Lucie apologises for being so easily tricked, but the Doctor is consumed by guilt for not saving the Keltans. Lucie is sad too, but she reminds him that he used to call Earth his ‘home’ too and there are still plenty of other planets that need saving. Reluctantly, and with nothing better to do, the Doctor sets to co-ordinates for Earth.