The Doctor finally takes Rose to her first alien planet, but by accident, as an external force has caused the TARDIS to materialise. The Doctor and Rose emerge and see cruel overseers armed with whips and futuristic guns forcing a slave gang to build a set of pyramids. Before they can do anything, they are captured by more overseers and forced aboard two hovering craft. Before they are separated, the Doctor promises to find his way back to Rose; however, when the overseers determine that he is not human, they gas him and transport him to another world in the system. When he awakens, a woman named Lazlee Flowers is there to orientate him to his new life. This entire solar system, Justicia, is a prison, and as nobody believes that it’s possible for the Doctor and Rose to have missed all of the warning beacons around the system, they have been found guilty of trespassing on Justice Alpha and sentenced to 25 years imprisonment.
As an alien, the Doctor has been sent to the Species-led Creative and Advanced Technologies House to serve his sentence by applying his alien knowledge to scientific research and development. The SCAT-house was originally created as a prison for non-human offenders within Earth’s jurisdiction, but as the Empire expanded and crime became rampant, the executives of Justicia hired out the other planets to serve as prison camps and testing centres. The Justician overseers are experimenting with different forms of punishment, such as hard historical labour, as the Doctor and Rose witnessed on Justice Alpha. Flowers assures the Doctor that Justicia is not run by monsters, but the Doctor warns her that he’ll show her a real monster if anything happens to Rose.
Rose has been transported to Detention Centre Six on Justice Beta, where she is taken off her shuttle by two guards -- the big, burly Norris, and the petite Blanc. She insists that this is a mistake, and Blanc promises to sort everything out with the Governor if Rose co-operates; however, she’s just calming Rose down until she can dump the troublesome newcomer into a room with four other hard-case teenage prisoners. The tough Kazta intends to show Rose who’s boss here by scalping her, but before she can do so, a trusty “blockwalker” named Dennel shows up and sends the others away, explaining to Rose that Blanc set this up to knock the fight out of her at the start. Rose learns from Dennel that this is the year 2501; most other prisoners in Justicia have been given brain implants to monitor their behaviour, but this borstal is being run without implants as an experiment. Dennel is about 20 years old, and he has already served 7 years of a 17-year sentence for what he claims was a minor charge. He turns Rose over to the warders, who strip her, shower her, give her baggy institutional clothing, and throw her in a cell with another young girl for the first night of her long sentence.
Flowers gives the Doctor a tour of the SCAT-house, which is buried deep beneath Justice Prime’s uninhabitable, frozen surface. The prisoners all have brain implants to translate the other prisoners’ languages and monitor their behaviour; if anyone even thinks about trying to escape or harm another prisoner, grey globs descend from the ceiling to envelop them. The Doctor has a choice of possible experimental workshops in which to participate; the hydroponics centre was closed down by Consul Issabel, who runs the SCAT-house, but other teams are working on ways to control solar flares and accelerate gravity, making it possible to terraform planets with tight inner orbits and travel instantaneously throughout the galaxy. Flowers then shows the Doctor to his temporary cell; until the Doctor is assigned a cell of his own, he is to be sharing a cell with two criminals from Raxacoricofallapatorius: Ecktosca and Dram Fel Fotch Heppen-Bar Slitheen.
The Doctor notes that the Slitheen have decorated their cell with photographs of other Slitheen in other species’ skin-suits, but they claim to be simple historians. After the Pasameer-Day Slitheen’s failed attempt to destroy the Earth, the Slitheen family fortunes went downhill; however, while researching their family’s criminal past, Ecktosca and Dram learned that their ancestors’ compression fields had been salvaged by the Earth governments and held for further study. The trail led them to Justicia, but although Consul Issabel agreed to help them recover their ancestors’ belongings, she betrayed them, framed them for breaking and entering, and had them imprisoned in the SCAT-house so the Justician Executive could tap their scientific ingenuity. The Slitheen claim to be waiting patiently while others of their family work to get them out, but the Doctor suspects that they’re working on an escape plan of their own.
Morning breaks on Justice Beta, and Rose gets to know her cellmate, Rizwana Mani, who was thrown into detention as a child when her mother cheated on her tax benefits. Riz tells Rose that the prison recently became mixed-gender as another of the Executive’s experiments, and warns her that she’ll get burnt if she gets too close to Dennel. Rose is assigned to peel potatoes in the kitchens, but the other prisoners taunt her and nearly start a fight before Blanc and Norris arrive to break it up. Blanc drags Rose off to answer to the Governor, but Rose smells something foul and sees a flickering blue light from the Governor’s office, and begins to panic, believing that the Governor is a Slitheen in disguise. However, when Blanc forces Rose into the office, she sees that the light is coming from a flickering desk lamp, although the Governor does appear to be a large man with a bad case of gas. The Governor orders Norris and Blanc out and questions Rose in private, convinced that she’s been planted in his prison to keep an eye on him; otherwise, it doesn’t make sense that she could be caught so easily after getting so deep into Justicia without detection. He insists that the prison is running smoothly, despite whatever she may have heard, and warns Rose that he’ll be keeping an eye on her. It may be the stress that is giving him digestive difficulties, but Rose is convinced that the fat, gassy man is in fact a Slitheen in disguise.
Morning breaks on Justice Prime, and the globs descend on the Doctor to march him to the workshop. There, the mammoth-like Yahoomer, the reptilian Blista, and a Sucrosian named Nesshalop are working on the accelerated-gravity project under Flowers’ supervision, using mindmitters to interface between their implants and the computer system. The Doctor promptly throws out all of the team’s work and suggests a radical new technique of accelerating gravity waves. Nesshalop picks up on his ideas and runs with them, but the equations that she and the Doctor generate are so complex that they burn out the computer, and the globs descend upon them both and punish them by draining their life essences. Flowers calls off the globs, having seen enough to understand that the Doctor has made a fantastic breakthrough -- but with the console damaged, she is unable to communicate with Nesshalop, and the Doctor refuses to tell her what she wants to know unless he gets to speak to Rose.
Dennel contacts Rose in her cell after lights-out, warning her to expect trouble from Kazta and telling her that the Governor will be having lunch in their canteen tomorrow in order to improve morale. Rose plans to prove that the Governor has been taken over by aliens so the Executive will start listening to her. Rose gets through the next morning without trouble, as Kazta intends to deal with Rose herself and has warned the other prisoners off. At lunch, Rose starts a food fight by flinging a glob of mashed potatoes at Kazta, and as the room erupts in chaos, Rose tackles the Governor and tries to unzip his forehead. Unfortunately, it turns out that he is just a fat, gassy human after all, and the guards pull Rose off the Governor and beat her unconscious.
Since the accelerated-gravity workshop has been put on hold, the Doctor is given a tour of the SCAT-house by the globs, and he sees his Slitheen cellmates building giant compression fields with which to contain solar flares. Still suspicious of his cellmates, the Doctor goes through their belongings when he returns to the cell, and discovers that they’ve secretly constructed personal compression fields for themselves; presumably, they intend to escape by using the compression fields to confuse their implants. Ecktosca and Dram return to their cell and catch the Doctor; the globs prevent them from attacking him, but they advise him to keep out of their business, and cryptically warn him that he’s being watched. Meanwhile, Flowers reports to Issabel and suggests that they reunite the Doctor with his colleague, whom he claims is an astrophysicist. The Senate is due to meet tomorrow, and Flowers desperately needs to be able to report some progress, as their last two meetings have been cancelled. Issabel agrees to consider the Doctor’s demands, and collects the damaged data core from the console to see whether it can be repaired without his help.
Rose wakes to find that she’s been locked up in solitary. Dennel contacts her from outside to tell her that the other prisoners are impressed by her attack on the Governor, but then retreats when he hears someone coming. It’s Norris, who is supposedly bringing Rose her dinner -- but to her dismay, the large man enters the cell, telling her that they have to talk. However, he’s not a Slitheen either; he’s the real undercover agent sent in to investigate reports of wrongdoing in the borstal. Prisoners and warders have been disappearing, the Governor is trying to cover it all up, and Norris hopes that Rose was sent in to get his report, since his contacts have been silent for the past five months. As he explains this to her, Blanc enters the cell -- and snaps Norris’ neck, telling the horrified Rose that Norris’ contacts are all dead. She then unzips her forehead, and Rose realises too late that compression field technology has advanced in the past five centuries and that the Raxacoricofallapatorians can hide inside skinnier frames.
Rose flees for her life, but realises that Blanc has deliberately left some of the borstal’s doors unlocked so her prey will work up a sweat of fear as Blanc stalks her. Fortunately, just as Blanc is about to catch up to her, the warders Jamini and Robsen arrive to collect Rose and take her to the Governor’s office; Consul Issabel has decided to let the Doctor speak to her as he requested. By the time Rose convinces them to look into her story, Blanc has rushed back to the cell and removed Norris’ body, and there’s no proof of her claims. Jamini takes Rose to see the Governor, but Robsen hears Kazta crying quietly in her cell; when he investigates, she claims that nothing’s wrong and that Blanc checked in on her some time ago. Robsen decides to visit Blanc’s quarters, and as he approaches, he hears her apparently speaking to another woman; however, when he enters, he finds that Blanc is alone. Blanc claims to have been listening to a recording that Kazta gave her; it sounds as though an alien is threatening to kill Kazta unless she does as instructed, but Blanc claims that Kazta just staged the incident to get attention. Robsen reports the incident to the Governor, but he brushes it under the carpet, desperately insisting that everything is under control in his prison.
The Doctor is contacted via his implant and told to leave his cell, but Ecktosca and Dram don’t hear the summons and fear that he is leaving of his own accord to tell Issabel about their escape plan. In fact, he’s being taken to talk to Rose, who is to be given one chance to prove her credentials as an astrophysicist. Flowers brings up a map of the Justicia system with which to check out Rose’s calculations, and the Doctor is surprised to see that, apart from Justice Prime, the planet’s orbits are almost perfectly circular and equidistant; it should be possible to use the entire system as a gravitational centrifuge. Flowers then primes Blista’s gravometer to verify Rose’s calculations, but finds that its readings for the system are off, and concludes that there’s a flaw in the gravometer.
When Rose is brought before the videolink, she warns the Doctor that she’s been attacked by a Slitheen, and although Issabel seems shocked by the news at first, she quickly recovers and dismisses Rose’s claim out of hand. The Doctor manages to convey the information Rose needs to know, using codes and innuendo that only she will understand; Issabel is suspicious, but before she can catch Rose out, alarms sound throughout the SCAT-house, and Issabel is forced to cut the interview short and order the Governor to ship Rose to Justice Prime immediately. The Doctor is marched back to his cell, where he learns that the Slitheen, fearing that he was going to betray them, have put their plan in motion and escaped. Not only has the Doctor missed his chance to escape with them, but now security will be stepped up, making it harder for him and Rose to break out themselves. Meanwhile, the Governor sends Rose to the shuttle bays, delighted to have rid himself of a troublemaker. Rose tries one last time to warn the warders of the danger they face, and although they don’t believe her, Jamini is a little worried that Rose is still making such wild claims even though she’s being released from the centre and no longer needs to stir up trouble.
While on her way to attend the Senate meeting, Flowers sees one of the Senators, Dr Meldow, entering the abandoned hydroponics centre, but he doesn’t respond to her calls -- and when she arrives at the meeting room, she finds it empty. Issabel claims that the meeting was cancelled due to the Slitheen’s unexplained escape, and insists that Dr Meldow is nowhere near Justice Prime. Worried, Flowers returns to her workshop, where the Doctor claims that the team is ready to start constructing their hardware, and requests that Flowers return his sonic screwdriver and arrange for the local gravity to be turned down within the workshop. He also claims to have double-checked the readings from Blista’s gravometer; they are accurate, and it’s the computer model that’s off. Somehow, the planetary orbits have become perfectly circular and equidistant, as if someone or something is turning the system into the perfect gravitational centrifuge and the Doctor’s team is just finishing their work for them.
The shuttle taking Rose to Justice Prime is halfway there when Rose smells smoke from the cargo bay. When the pilot investigates, Dennel leaps out of hiding and knocks him out with the fire extinguisher. He assures Rose that he was in perfect control of the fire he set, and reveals that he was imprisoned for arson. He hid out in the corridor last night after speaking to Rose and saw everything that happened next, and before Blanc returned for Norris’ body, Dennel took his passcard and used it to board the shuttle, intending to rescue Rose. However, since she was just being taken where she wanted to go, he’s just made matters worse for her. Rose then discovers that the pilot needs to log onto the console regularly to prevent the controls from shutting down, but when she and Dennel try to drag the pilot’s unconscious body into the cockpit, they discover that he is wearing a skin-suit...
Rose uses Dennel’s lighter to burn the hand away from the rest of the skin-suit so she can use the palm print on the flight console, but the flame interacts with the compression field’s gas exchange, causing the alien’s real body to grow to full size and shred the rest of the skin-suit. Rose and Dennel retreat into the cockpit with the suit’s hand, which Rose slaps onto the flight console, cancelling the lockout countdown. However, the pilot then wakes up, and when Rose pushes past Dennel to lock the cockpit door, she accidentally knocks him onto the flight controls. The shuttle spirals off-course straight towards Justice Delta, home of the Executive. As the shuttle enters Justice Delta’s atmosphere, Rose and Dennel are forced to let the pilot back into the cockpit so he can steer them to safety, but despite his efforts, the shuttle crashes into a monitoring platform and goes down in the jungle.
The worried Flowers confirms that the planetary orbits have indeed changed, and sets off to report her findings to Issabel. But as she approaches Issabel’s office, she hears the Consul crying out in pain -- and sees her stripping off her skin-suit to emerge into the open air. Her real name is Ermenshrew, and she’s just felt the pain of her cousin Clem Sel Hotch as his shuttle crashes on Justice Delta. As Flowers listens from hiding in the corridor, Ermenshrew contacts her patriarch, Don Arco, who insists that they had no choice but to transport Rose to Justice Prime by shuttle; the family have been using “pathways” to travel between the planets, but the pathways are still too unstable for humans to survive such a journey. However, it now seems that the Doctor and Rose’s expertise is no longer necessary; Ermenshrew’s uncle has salvaged the data from the damaged core, and the family is ready to make their move and seize complete control of the Justicia system.
Ermenshrew then smells Flowers’ presence and pursues her down the corridor. The terrified Flowers runs into the Doctor, who tries to help her escape; however, since he has a prisoner’s implant, the globs keep descending and interfering with his attempt to flee. Flowers is able to use her authority to order the globs off him, but when she returns the Doctor’s sonic screwdriver and he uses it to break into the systems hub, the globs descend and punish him for this “sabotage” by draining his life essence. He is forced to hand the sonic screwdriver back to Flowers, and the globs let go; however, he’s recognised one of them, which has a distinctive bite, and realises that he’s encountered it elsewhere in the facility. He thus orders Flowers to use the sonic screwdriver on the systems controls to trigger a zero-gravity field throughout the SCAT-house. As Ermenshrew tries to squeeze through the small door after them, she tells the Doctor that his friend has gone down in flames on Justice Delta; however, before she can reach Flowers, the zero-grav field kicks in, and the Doctor and Flowers float up to the ceiling, out of Ermenshrew’s reach. The Doctor had worked out that the globs must be able to travel freely throughout the SCAT-house to cluster wherever trouble breaks out, and indeed, the high ceiling turns out to be a network of tunnels for the globs. The Doctor and Flowers scramble to safety in the tunnels before the gravity returns to default Earth-normal, and to their relief, the globs don’t respond to their presence. For the moment, they’ve escaped from Ermenshrew, and the Doctor now vows to get to Justice Delta and find out what’s happened to Rose.
Rose’s shuttle has crashed in the jungle, but is still intact, and its occupants have survived. The injured pilot asks Rose to call for help, claiming that it would rather be in prison than dead, but takes offence when she refers to it as a Slitheen; it is from the same planet, but is of the Blathereen family. Rose sends out a Mayday as requested, and learns too late that she’s been tricked; the Blathereen have already killed off the Executive and seized control of Justice Delta, and now they’ll be coming for her. Rose and Dennel flee, seeking shelter in the monitoring platform they brought down with them -- but on the way, they stumble into a clearing containing the bodies of the prisoners and warders who disappeared from Justice Delta. Rose spots Norris’ body and theorises that Blanc has been indulging in her passion for the hunt and disposing of her victims here -- but that doesn’t explain how she transported the bodies to Justice Delta, or why some of the bodies have been turned inside out.
Back on Justice Beta, Robsen and Jamini learn that there is to be a massive influx of new teenage prisoners, and to make room, male and female prisoners will be made to share cells. Even the Governor questions this bizarre directive -- at least until Blanc arrives at his office with another Blathereen, who kills the Governor and takes his place. All over Justicia, human overseers are being killed and replaced by the Blathereen, who are now ready to launch their main project. Meanwhile, Kazta contacts Riz and claims that Rose was telling the truth about the monsters; the one inside Blanc threatened to kill Kazta unless she told Robsen that Blanc was with her at the time Rose was supposedly attacked. Kazta tried to tape their conversation, but Blanc found the tape and confiscated it. One of Kazta’s cronies, Maggi Jalovitch, claims to believe their talk of monsters, and although Kazta is too frightened to do anything about it, Maggi, uncharacteristically determined, decides to tell Robsen herself.
The Doctor uses his sonic screwdriver to light his way through the tunnels, and while doing so, he and Flowers run into two very large globs with suspicious zip fasteners: Ecktosca and Dram, who started a fight in their cell and activated their compression fields when the globs descended upon them. As the Doctor suspected, the compression field interfered with the Slitheen’s implants, and the globs retreated to the ceiling while Ecktosca and Dram hung on. The Slitheen reveal that they’ve been aware of Ermenshrew’s true identity all along, but said nothing because they regard this matter as a private family feud; the Blathereen family fortunes rose when the Slitheen’s fell, and Ermenshrew really did betray them and lock them up so she could use their brainpower for her family’s project. The Blathereen have been using a lodestone on their mothership and a gravity warp somewhere on Justice Prime to nudge the system’s planets into more perfect orbits, fine-tuning the centrifuge and stabilising the system of wormholes they’ve been using to smuggle their family into Justicia. Now, thanks to Flowers’ team, they’re ready to put their scheme into operation -- whatever it is.
Flowers realises that “Dr Meldow” must have been a Blathereen -- Ermenshrew’s uncle, who collected the damaged data core. This means that there must be a wormhole terminal located in the aquaculture compound , which is why “Issabel” shut down the hydroponics experiments. The Doctor tries to light the way through the tunnels to the compound, but the sonic screwdriver begins to run out of power -- and then the globs start attacking them, as Ermenshrew has reached the controls and programmed the globs to attack everyone up in the tunnels. The Doctor, Flowers and the Slitheen leap out of the tunnel in the aquaculture compound, and the globs gently lower them to the ground; there, they release Flowers, since she does not have a prisoner implant. However, they continue to attack the Doctor, and when Flowers tries to order them off she finds that her authorisation has been cancelled. The Doctor spots a feed line that’s drawing frozen nitrogen down from the planet’s surface and mixing it with ammonia to fertilise the plants in the compound; on his instructions, Flowers opens it up with the sonic screwdriver, and a blast of frozen nitrogen freezes the globs enveloping the Doctor, enabling him to shake them off. In the confusion, however, Ecktosca and Dram have vanished, having presumably found the wormhole entrance and escaped.
As the Doctor recovers, he points out that the nitrogen blast was more powerful than expected, which suggests that the Blathereen’s gravity warp controls are on the surface, generating a gravity field that forced the frozen nitrogen down the feed at high pressure. He then spots a poppito tree, a plant native to Raxacoricofallapatorius, apparently growing in the centre; before he can investigate, however, Ermenshrew arrives and reveals that, even if the Doctor locates the portal, only a native of her planet can activate it. Ermenshrew has decided that she no longer needs the Doctor; she’s been forcing Flowers’ team to complete their work and has ripped off one of Nesshalop’s eyes as a demonstration of her power over them. Before she can kill the Doctor, however, he claims to have sent a hyper-destructive pulse up through the nitrogen feed to destroy the gravity warp. Ermenshrew is forced to investigate, despite the possibility that he’s bluffing, and as soon as she’s gone, the Doctor places Nesshalop’s eye in a nutrient tray, hoping to reattach it later. Ecktosca and Dram are then forced out of hiding by the growing stench of ammonia; they were hoping to escape on their own, but are now forced to activate the portal and flee. The Doctor and Flowers follow, despite the risk that the wormholes may still be too unstable for humanoids to survive the journey.
On Justice Beta, Robsen decides to speak with Blanc, but finds Maggi in Blanc’s room. She claims that Blanc dragged her here to stop her from speaking to Robsen, and shows Robsen a black disc that she claims Blanc has been using to transport people through space. Robsen is sceptical, until Maggi pushes him onto the disc and he is transported through space to Justice Delta. He materialises in the clearing with the unnaturally twisted bodies; Blanc had been transporting human subjects to test the stability of the wormholes, but most of the humans she sent through were torn apart by the stress of the journey. Meanwhile, back on Justice Beta, Maggi contacts Kazta and Riz and reveals that she’s stolen Robsen’s access pass. When the Governor and Blanc enter the canteen, Maggi unexpectedly appears behind them as if she’s popped up out of thin air, and unzips their skin-suit disguises before they can react. The warders and prisoners begin to panic, but Maggi whips up her fellow prisoners into a mob, and the prisoners descend upon the two Blathereen, tear them apart, and follow Maggi to the transport disc to hunt the other monsters.
The Doctor and Flowers survive their journey and find themselves on Justice Delta with the Slitheen. The Doctor uses his sonic screwdriver to generate a repulsion field around the teleport disc so that Ermenshrew will be diverted if she tries to follow them; however, the screwdriver’s power source is running low, and he fears that this will cause unwanted side-effects. When he succeeds, he lets out a triumphant whoop, and the Blathereen burst in, capture him and his allies, and march them to an audience with Don Arco. The Blathereen patriarch is squatting in a lecture hall filled with the smoke from burning incense; he was once forced to hide from the authorities in a toxic dump, and his lungs never recovered.
Don Arco laughs to see that the Slitheen have been recaptured, and informs the disappointed Doctor that the Slitheen did indeed intend to recover their ancestors’ technology in order to get back into the family business. However, the Blathereen have set their sights higher than Ecktosca and Dram ever imagined. Using the stable gravity centrifuge and the hardware developed by Flowers’ team, they intend to transport the entire Justicia system through space and use the solar compression fields to turn the systems’ stars into giant flamethrowers with which to incinerate planets; they will then take the planets apart to sell as fuel, and leap back to Justicia space with nobody the wiser. The prisoners in the system will be used as slave labour to refine the radioactive material and build storage facilities for it, and the mixed-gender prisoners in the borstals will be used as breeding stock to ensure a plentiful supply of human slaves. The Blathereen have already replaced Norris’ former contacts, the members of the EarthGov committees in charge of monitoring Justicia, and as long as EarthGov remains unaware that anything is amiss, they will keep shipping prisoners to Justicia to replace those that the Blathereen have worked to death.
On Justice Delta, Rose and Dennel reach the damaged monitoring platform, which is wobbling back and forth as its damaged anti-grav motors try to lift it back into the air. Inside, they find a CCTV system that confirms the Blathereen are in control of the entire Justicia system; they also see Robsen materialise back in the clearing, and realise that if it’s being monitored, the Blathereen will have seen them there. Rose and Dennel return to the clearing and tell the stunned Robsen what’s been happening, and then flee when they hear a loud crashing sound, presumably the sound of the Blathereen approaching. As the crashing noise gains on them, they nearly run into a patrol of Blathereen coming from the opposite direction; the crashing sound turns out to be the monitoring platform grinding its way through the jungle, presumably being pushed along by the damaged anti-gravs. Rose, Dennel and Robsen climb on board, and moments later the platform hits something and tumbles end over end, swinging the door up away from the ground and running over some of the Blathereen. Temporarily safe inside the platform, Rose then spots the Doctor on one of the CCTV monitors, and when she turns up the volume, she, Dennel and Robsen hear Don Arco explaining the Blathereen plan.
Ermenshrew bursts into the lecture hall in a rage, having felt her daughter’s death at the hands of the rioting prisoners on Justice Beta. Don Arco insists that they keep the Doctor alive to use his intelligence, but then monitoring platforms begin to fall from the sky and crash into the complex. In the confusion, Ecktosca and Dram escape, while Don Arco and Ermenshrew teleport back to the SCAT-house. As the Doctor and Flowers flee outside, Rose’s monitoring platform hurtles out of the forest, but she, Dennel and Robsen leap out to safety moments before it hits the complex and goes up in flames. The Doctor realises that this is his fault; when he generated the repulsion field to divert Ermenshrew, he had to set up an attraction field in order to balance it out, but because his sonic screwdriver was running low on power, it accidentally made the attraction field too powerful -- and it interacted with the monitoring platforms’ anti-gravs, pulling them into the complex.
A mob of enraged Blathereen emerge from the burning complex, and when the Doctor and his friends flee towards the jungle, they run into the Blathereen patrol that was sent out to find and kill Rose. At the last moment, however, the mob from the borstal pours out of the jungle and sets upon the Blathereen patrol, beating them to death. Maggi tries to whip up the other kids into a frenzy and send them after the other Blathereen, but Kazta and Riz then spot Rose, who has become something of a celebrity amongst the borstal kids. Rose explains that they have to destroy the other buildings so the Blathereen will be unable to use them to navigate the Justicia system through space, and Dennel agrees to use his skills as an arsonist to organise the mob. Before going, he gives Rose one of his many lighters so she will remember him. Rose then leads the Doctor, Flowers and Robsen to the portal in the jungle, but once there, they realise that only a Raxacoricofallapatorian can activate it... which begs the question of how Maggi managed to use it to transport the mob here. The question is answered when Maggi arrives in the company of Ecktosca and Dram, who reveal that she’s really their auntie Callis. Callis smuggled herself into Justicia, killed the real Maggi and took her place in order to use the borstal kids as an army to get through the Blathereen to her nephews.
The Slitheen are now ready to put this mess behind them, but the Doctor convinces them that they’ll never live it down if the Blathereen succeed in their plans, and the Slitheen reluctantly agree to help him defeat their rivals. The Slitheen thus prime the teleport disc and take the Doctor and his allies back to Justice Prime, where Ecktosca, Callis and Robsen go to destroy the gravity warp on the surface while the Doctor, Rose, Flowers and Dram confront the Blathereen. However, the Blathereen have sealed themselves inside the accelerated-gravity workshop, where Yahoomer, Blista and Nesshalop have been forced to complete their work. With the quantum computers on Delta destroyed, the Blathereen will be unable to steer Justicia about with any precision -- but they intend to take it for a joyride anyway and incinerate a few innocent planets just for the fun of it. They are unaware that Ecktosca and Callis are attempting to destroy the gravity warp that is stabilising the planet’s orbits -- and if the Slitheen succeed and the Blathereen then switch on, the entire Justicia system could be torn apart by the gravitational turbulence.
Ecktosca and Callis climb up to the gravity warp on the planet’s surface while Robsen stands guard in the aquaculture compound, but find that, due to the Doctor’s earlier bluff, Ermenshrew has placed globs on guard with instructions to kill anyone who sets foot in the gravity warp controls. Meanwhile, Robsen sees Nesshalop’s eye winking a warning at him, and thus avoids an attack by a Blathereen that was creeping up behind him. He tells the Blathereen that a horde of humans are attacking the gravity warp, and when the Blathereen climbs up to investigate, some of the globs that were attacking Ecktosca and Callis turn on the Blathereen, enabling the Slitheen to retreat. They refuse to return to their work and risk their lives, but Robsen, fearing that the Blathereen might test their incineration engine on the star system where his children live, volunteers to climb up the ladder and let the globs attack him, drawing them off while Ecktosca and Callis resume their work. The Slitheen successfully sabotage the gravity warp system, and, despite himself, Ecktosca drags the unconscious Robsen to safety before he and Callis flee.
Rose suggests setting a fire to distract the Blathereen, which inspires the Doctor to come up with a related idea. He orders Rose and Flowers to generate an anti-gravity field in the workshop in exactly fifteen minutes, while he and Dram return to the solar workshop, tap into the energy from the gravity accelerator and increase the power of the solar compression fields. Meanwhile, Rose and Flowers discover that Ermenshrew has now padlocked the gravity controls shut, and by the time they pry it open, there’s no time to localise the anti-gravity field. Flowers thus generates a field throughout the entire SCAT-house, and as she and Rose float in mid-air, Rose nervously thumbs Dennel’s lighter -- and sees for herself what the Doctor’s plan is.
The Blathereen have finished installing their controls and are ready to test-drive their new incineration engine by destroying the New Washington system. But before Ermenshrew can activate the system, the anti-grav field kicks in -- and as Rose has just learned, smoke and flames don’t rise in a zero-gravity environment. As Don Arco floats helplessly up into the clear air near the ceiling, smoke pools around his incense candles and snuffs them out. One of the Blathereen technicians floats into the sputtering candles and is burned, and when he stumbles out of the workshop, he inadvertently lets the Doctor in. The Doctor confronts Don Arco, threatening to let him choke unless he surrenders, but when Don Arco gives in, the furious Ermenshrew kills her patriarch and declares herself the new head of the family. The gravity in the SCAT-house resets to default again, but rather than kill the Doctor, Ermenshrew decides to activate the engine and let the Doctor watch as an innocent solar system is destroyed. The Doctor warns her that the gravity warp has been destroyed and that he’s boosted power to the solar flare compression field and aimed it at the Blathereen mothership. However, Ermenshrew does not believe him, and she activates the controls. The Doctor and the rest of Flowers’ team flee from the workshop as the Blathereen mothership is crushed out of existence by the compression field, and the feedback blows up the guidance controls, killing Ermenshrew and the other Blathereen technicians in the gravity workshop.
In the aftermath, Flowers and Robsen find themselves in charge of tracking down the surviving Blathereen and getting Justicia back under control. Ecktosca and Callis were apparently incinerated when the gravity warp exploded, but the Doctor nominates Dram, the rest of Flowers’ team, and the borstal kids for early release. The sole surviving Blathereen from Justice Prime requests permission to burn Don Arco’s corpse on a funeral pyre, but then leaps into the flames and vanishes from sight along with his burning patriarch. The Doctor and Rose return to the TARDIS, free to go now that the centrifuge is no longer drawing their ship down, and leave the people of Justicia to sort out their problems. Soon after they’ve gone, however, Dram is reunited with Ecktosca and Callis, who hid inside the bodies of Don Arco and the Blathereen technician and used Callis’ personal teleport to escape from the middle of the funeral pyre. Now, everyone believes that Ecktosca and Callis are dead and that Dram is in mourning -- giving them an opportunity to disassemble the gravity warp, figure out how it works, and use it to jump-start the Slitheen family’s triumphant return to crime.