The TARDIS is struck by a psychic blast, equivalent to a telepathic virus, and Anji is affected by the attack and falls into a coma. The Doctor tries to find a world where she can receive suitable medical care, but when he and Fitz emerge from the TARDIS with her they find that they have landed in the middle of a raging dust storm. Fitz is struck by a flying rock and injured, and the Doctor digs a hole where he can shelter while the Doctor takes Anji to the nearby city for help. But as the Doctor approaches the city, he realises that it’s also approaching him; it’s a gigantic earth-moving machine, miles in length, and he’s left Fitz buried in its path...

This is Ceres Alpha, in the year 2873; the planet is close to Earth-normal, and Gaskill Tyran’s WorldCorp has contracted to terraform it so human beings can live outside for the first time in centuries. But all is not well. Comptech engineer Veta Manni and her husband Josef have not recovered from the trauma of their baby’s stillbirth two months ago. Veta is suffering from depression, and Josef has tried to lose himself in his work only to find that software glitches in the city mainframes are multiplying at an alarming rate and aren’t correcting themselves as they should. When he returns home from a hard day’s work to find that Veta has hacked into the medical systems, stolen a hologram of their dead baby and placed it in the nursery, he fears that her grief has driven her mad. But it’s far stranger than that; there were problems with the pregnancy before the birth, Josef and Veta were not permitted to touch their dead child due to “quarantine regulations”, and a man in a suit -- not a doctor -- was watching at the birth. Now, there are small alien creatures penned up in a secret annexe behind Medicare Central, and Dr Pryce can sense the hatred they feel for him. His associate Mij Perón is watching as the stress of the experiments they are conducting slowly drives Pryce mad...

The city’s archaeologist, Danyal Bains, is starting to realise that his position is token at best. He has found evidence that the planet was once home to an alien civilisation, but Tyran refuses to halt the terraforming process, even though Bains’ excavation will be destroyed when the city runs over it. Bains does all he can to get word to Earth Central, but is blocked at every turn; Tyran has even bought out those whom Bains had considered close friends. Bains retreats to the bar at For’ard Obs, where he turns down the advances of the beautiful Carly Dimitri while he considers how to stop Tyran. Dimitri is forced to report failure to Tyran, who punishes her by using a mind probe which amplifies her fear of him and feeds it back to her mind. Realising that Tyran will kill her if he has no further use for her, Dimitri tries to kill him with the mind probe. She fails, and he burns out her mind and orders his bodyguard Zach to dump her body over the side of the city.

The city sensors detect two life signs in the path of the machine, and Captain De-ann Foley and her team are sent out to rescue the new arrivals. The Doctor and Anji have both been battered senseless by the storm, and Foley takes them to Medicare without realising that a third person is out there somewhere. The man appears to have extensively modified his internal biology, and the woman’s brain is giving off peculiar readings; however, Pryce is near the end of his tether and is sure that Doctor Domecq from Earth Central must have been brought down by the storm after arriving early. When the Doctor awakens, he plays along with Pryce’s error in order to get help for Anji and Fitz. Before going, the Doctor suggests an “experimental” treatment for Anji’s condition -- in fact a common remedy which hasn’t been invented yet. Perón, who is in fact a colonel in the Earth military, isn’t convinced that the Doctor really is who he says he is, but he seems to know what he’s doing, so she plays along with him for the moment.

Bains tries to steal his impounded chopper and get back to his dig before the city destroys it, but the chopper has been bolted down, and when he fires its jets he seriously injures a trooper who was trying to break in and stop him. As the Doctor and Foley leave the city, they see the soldiers drag Bains out of the chopper and beat him senseless; the Doctor isn’t impressed, but he sets the problem aside for the moment out of concern for Fitz. Sadly, there’s no trace of Fitz outside; it appears that the city has already run over him. Upset, the Doctor has Foley collect the TARDIS and take it to the docking bay while he checks on Anji. As her condition is improving, and as Pryce is obviously desperate to pass on his problem, the Doctor agrees to see the “creatures” which Pryce mentioned. He is horrified when Pryce leads him through a secret panel to a set of cells where half-human, half-alien children have been jailed while Pryce and Perón conduct vile experiments upon them under orders from Earth Central. The Doctor, enraged, plays a sleight-of-hand trick with one of the creatures, and Pryce, who had only sensed anger and hostility from them, is stunned when the child responds with wonder and fascination.

Tyran summons the Doctor to his office, but finds it impossible to get a handle on his sense of moral outrage. He is surprised when the Doctor refuses a bribe to keep the situation quiet; even a rival agent should at least have played along. But even if the Doctor isn’t the real Domecq, he obviously has enough knowledge and expertise to learn the truth about the children, and Tyran thus decides to let him work alongside Pryce. But Bains then bursts into the office, having tricked and overpowered his guards at the prison, and intending to force Tyran to shut down the city before it destroys his dig. Before the Doctor can learn more about Bains’ claim to have found relics from an ancient alien civilisation, Foley and her team break in to take Bains out, but the Doctor saves him by taking a guard’s gun and putting it to Tyran’s head. Surprisingly, Tyran orders Foley to release Bains and see that he gets proper medical treatment before being returned to his quarters; Bains decides not to push his luck. As the Doctor turns over his gun, Tyran warns him to take care; Tyran was abandoned in the gutter as a child, and he has done and will do whatever it takes to claw his way to the top and stay there. He sends the Doctor back to Medicare Central, privately warning Perón to keep an eye on him.

Oddly, Fitz emerges from the soil in a different part of the planet. Ayla Damsk, a surveyor at fieldbase Gamma Twelve, finds him while collecting soil samples and takes him back to camp for medical attention. However, not everybody is so accomodating; the planet seems to be resisting the attempts to terraform its soil, and the frustrated workers are looking for a scapegoat. Rassel Jörgan in particular is convinced that Fitz must be an agent for a rival company, who was buried in a landslide while trying to poison the soil. Fitz realises that he’s in danger, but is caught trying to slip away and is interrogated. When he mentions that his friends were heading for the city, Ayla calls in to confirm this; her call is directed to Perón, who orders her to question Fitz and find out whether the Doctor really is Domecq. Jörgan and his men try to beat a confession out of Fitz, but Ayla bluffs by holding an empty gun to Jörgan’s head, forcing him to let Fitz return to sickbay. There, Ayla tells Fitz that she saw innocent people die in witch hunts when the terraforming programme went wrong on Gildus Prime; she won’t let that happen again.

While staring at the hologram of her dead son, Veta realises that the shadows don’t match the lighting in the quarantine bay, and realises that the record is a fake. At first Josef fears for her sanity, but when she probes further she discovers that Medicare Central was taken over by the military shortly after the birth, and that they’re still in charge. Also, the man who watched the birth was Gaskill Tyran himself. Veta determines to learn the truth from Pryce, but Pryce has already been visited by the Doctor, who has confronted him about his treatment of the children. Pryce had also regarded them as children until Perón insisted that they were alien monsters, not to be trusted; after that, he had to force himself to believe that she was right in order to carry out the experiments he did, and they sensed his fear and hatred and fed it back to him. He now realises that they are children after all, and what he has done to them is unspeakable. When Veta arrives at Pryce’s quarters she finds that he has committed suicide.

The Doctor spends the day analysing Pryce’s work, and files his notes under a safeguard which causes them to vanish when Perón tries to access them illicitly. In the meantime, the Doctor goes to Bains’ apartment, and tries to convince Bains to take him to the dig; he’s certain that the prior inhabitants of Ceres Alpha have some connection to what’s happening now. The scars of his recent failed attempt are too fresh in Bains’ mind, however, and by the time the Doctor finally convinces him to try, it’s too late; Foley has tracked the Doctor down, to take him to Tyran to make his report. Bains returns to For’ard Obs, where he drinks and remembers his late wife Jasmine, who abandoned him for a rich husband only to contact him once more, in a panic, thirty years later. When he finally responded to her call, he found her huddled in a closet, in a catatonic state from which she never recovered. Meanwhile, Foley, who is starting to feel a grudging admiration for the Doctor, is shocked when they walk into Tyran’s office to find that the real Doctor Domecq has arrived at last...

Anji awakens at last, and while recovering she has a vision of a small alien child pleading for help. Perón discovers that Anji has vanished, and tracks her to the roof, where she finds that Anji has somehow formed a psychic bond with the alien children, broken into the cells and tried to rescue one of them. Private Danes takes Anji back to the medical centre while Perón takes the child back to the cells, but Perón fears and hates the children as much as Pryce had, and she’s too rough with her charge. As she strikes out at the child, Danes forces Anji onto her bed and tries to inject her with a sedative -- and all of the syringes in the medical bay fly through the air and plunge into his body, killing him. When Anji recovers from the shock, she takes Danes’ com-badge and uses it to find her way to the VIP suites, believing that this is where the Doctor is likely to have ended up.

In fact, the Doctor is being tortured by Tyran, while the disinterested Domecq and the appalled Foley watch on. Tyran tries to use his mind probe to force a confession out of the Doctor, but fails; even the Doctor doesn’t seem to know who he really is... Frustrated, Tyran sends the Doctor to prison, and shows the alien creatures to Domecq. Meanwhile, Veta hacks into Perón’s private computer, causing it to break down, and redirects Perón’s maintenance call to herself so she can access the comp directly and download Perón’s private data into her own comp. When she gets the chance to look it over, she discovers the truth; the alien-human hybrids have been held prisoner for two months, growing at an advanced rate until they appear to be three years old. For all that time, they’ve been subjected to vicious experiments to test the extent of their telepathic powers; for example, Pryce or Perón would torture a child in one room while observing the reactions of a child in another room. Veta vows to kill Perón for what she’s done, but at that moment she suffers a psychic shock, as do the Doctor and Anji, elsewhere in the city. Tyran has just used his mind probe to kill one of the children.

Ayla gives Fitz the keys to her landbug so he can flee to the city, but he is recaptured before he can get to it. Fortunately, reinforcements arrive to take him in for questioning, and Fitz breaks free while the workers are distracted by their arrival. He shoots one of the camp’s fuel tanks to create a distraction, and steals some explosives as insurance. He is nearly recaptured again, but Ayla uses a flare to distract the troops and they flee together, using the stolen explosives to disable the pursuit vehicles. On their way back to the city, Fitz explains who he really is, but Ayla doesn’t believe him until they reach the city’s docking bay to find the TARDIS waiting where Foley left it. Fitz lets Ayla inside, and takes a much-needed shower while she gapes at the TARDIS interior.

Anji is attacked by a rat in the city maintenance shafts, but manages to escape as the city’s power systems begin to fail. She disguises herself as a cleaner and reaches the VIP suites, where she finds the Doctor’s coat in an office with blood stains on the carpet. She hides when Tyran and Domecq arrive, discussing the children; Tyran has proven that they can be killed, and it appears that the surviving children are now attacking the city’s mainframes telekinetically, causing the software glitches which are taking the city to the point of total shutdown. Tyran wants them dead, but Domecq wants them alive so Earth Central can learn how to utilise their psychic powers. Domecq plays on Tyran’s paranoia by suggesting that the children were planted by a rival corporation; and since the Doctor and Anji apparently showed signs of biological and psychic modification, just like the children, this implies that they were sent by the company responsible for the subtle attack. Anji has heard enough and tries to flee, but she is stuck in a lift when the power fails and is attacked by rats once again. She escapes from the lift, only to be recaptured by guards and sent to Tyran for interrogation.

Perón sends Foley to find out why Pryce hasn’t reported in, and Foley finds Pryce dead -- and evidence that someone was in his room shortly afterwards. She presents her report to Perón, who recognises one of the passers-by on the surveillance cameras as the comptech who “repaired” her com earlier that day. When she sees Veta’s name, she realises that she is the mother of one of the alien children. Veta has just learned of the secret annexe from the stolen files, but before she and Josef can set off to investigate, Foley and her team arrest them. But with all of the confusion caused by the power failures, the guards neglect to search them before throwing them into a makeshift cell; Veta is thus able to use her remote control to generate a hologram of herself and Josef, distracting their guard long enough for them to overpower him. Meanwhile, Perón files a report on the successful conclusion of the Pryce Experiment.

The security monitors in the prison fail, and when the Doctor’s guard checks on his prisoner, the Doctor overpowers him and escapes. He rescues the children from the medical annexe and takes them to Bains, who finally agrees to take him to the dig to look for answers. The alien children finally seem at peace upon arriving at the dig, and Bains watches in fascination as they give themselves names pulled from the Doctor’s mind -- the names of T.S. Eliot’s Practical Cats. They name the Doctor Old Deuteronomy. The Doctor, still grieving for Fitz’s apparent death, shows the children how to conduct a funeral for the dead child Bustopher Jones, but once buried in the soil the child returns to life. Foley and her team arrive with orders to kill them all, orders which Foley will reluctantly carry out despite her respect for the Doctor; but just as the soldiers are about to fire their weapons blow up in their hands, killing them all. The Doctor realises that the children caused this to happen, and tells them that they should simply have jammed the guns; all life is precious.

Tyran tortures Anji with the mind probe, drawing the image of the TARDIS out of her memory. He and his men try to break in, but fail, and Tyran loses his temper and shoots Anji; however, he regrets his rash action at once, and has Perón take Anji to the medical bay for treatment. The Doctor and Bains then return to the city, and the Doctor fires his chopper’s weapons over the guards’ heads, forcing them to take shelter while the Doctor and Bains get the children into the TARDIS. There, the Doctor is reunited with Fitz, but the children collapse, dying, and the Doctor realises that when the TARDIS doors closed their link to the planet was broken. Ceres Alpha is alive, but it suffered from a catastrophe which wiped out all life on the planet; it has been trying to regenerate itself ever since, but WorldCorp’s terraforming has interrupted the process. Bains’ dig disturbed the psychic residue of the planet’s original inhabitants, causing mutations in human embryos at a certain stage in their development; the children are not threats, but emissaries from a sentient biosphere which is trying to communicate with the invaders threatening its rebirth. Inside the TARDIS, they are cut off from the rest of the biosphere, and the Doctor is thus forced to open the doors, allowing Tyran’s troops to enter the TARDIS and arrest them all.

The children are taken to Domecq, and as they appear to be dying he decides to vivisect them to find out what makes their brains different. Meanwhile, Perón revives Anji, who feels the pain of the children and pleads for mercy. Perón remains convinced that the children are evil, and she accuses Anji of trying to get inside her mind and drive her mad just as the children did to Pryce. Before she can stab out Anji’s staring eyes, Veta arrives and guns her down. Veta and Josef free Anji, who tries to lead them to the children before it’s too late.

The Doctor tries to explain the situation to Tyran; the psychic birth trauma as the children were born was so powerful that it reached into the Time Vortex, striking down the TARDIS and affecting Anji, the closest person the children could find to a mother. Tyran doesn’t believe the Doctor, and threatens to shoot his friends, starting with Fitz, until the Doctor confesses that he was sent by a rival company. The Doctor cries out in protest at the same time that Perón attacks Anji and Domecq starts to cut open a child’s head -- and the children lash out, telekinetically flinging Domecq’s scalpels through the air and impaling him. At the same time, Tyran’s mind probe turns on him by itself, and his past memories are flung up onto the office wallscreens for all to see. As Tyran collapses and dies from the accumulated horror of all the things he’s done to claw his way into power, Bains sees a memory of Tyran killing Jasmine, and realises that Jasmine was the mother who abandoned Tyran to the rats; which means that Bains has been Tyran’s father all along.

Anji, Veta and Josef arrive moments too late, as the children finally die from the shock and strain of their recent exertions. But the Doctor rushes them all to the choppers and pilots the children back to the dig, where he buries them in the soil of the planet. Just as before, direct exposure to the planet’s biosphere causes the children to return to life. And, having saved the children who called for them, the Doctor, Anji and Fitz depart.