The Doctor and his companions travel to the planet Yemaya 4 to investigate a mysterious outbreak of psi powers amongst the human colonists. The Doctor and Dr Byerly soon determine that the colonists are falling victim to a virus, which may be generating new powers or triggering their own latent psi talents. While analysing a blood sample, the Doctor accidentally pricks himself with an unsterilised needle, and alien memories begin to overwhelm him as the viral RNA begins to overwrite his own. He shuts himself away in a virtual reality environment while his mind purges itself of the alien memories -- but when he recovers, he finds that he’s forgotten what the alien memories were, and is thus no closer to a solution. Nevertheless, the disease is clearly too elaborate to be natural, and the Doctor comes to suspect that the colonists have deliberately been infected by Dione-Kisumu, the corporation which sponsored the colonisation, as guinea pigs in an illegal experiment.

Tensions are growing between the newly affected humans and the unaffected colonists, who fear the “danger” posed by telepaths who can read their thoughts and telekinetics who could theoretically kill with a thought. Chris is beginning to develop telepathy, but he doesn’t want to admit this to Roz, who distrusts telepaths and is fiercely protective of her privacy. He thus doesn’t tell anyone when he begins to dream of fire and hears a voice in his head, calling him into the nearby forest. Meanwhile, Benny accompanies the colony xenobiologist and archaeologist to the alien temple in the nearby forest, where she finds graffiti written in a language influenced by the extinct Ikkaban species. The Ikkabans are known to have worshipped a universal symbol of death and rebirth known as the Turtle, and they deliberately sacrificed themselves in fire, one at a time. Benny also finds a piece of metal shrapnel from an Earth ship, possibly one of the robot drones which surveyed Yemaya 4 before the colony was established here.

Dot Smith-Smith, who has been deaf since birth, is infuriated when she begins to develop telepathy and becomes overwhelmed by the voices in her head. The Doctor, meanwhile, sets the colony’s AIs to work searching for clues in the mainframe, and then confronts Chris, having realised that Chris is becoming telepathic. Chris collapses in tears as he tries to resist the call in his mind, but when the Doctor tries to read Chris’ mind he realises that the memories of fire are Chris’ own -- they’re the memories of the flitter crash in which he was involved when he and Benny first met. Before the Doctor can do anything for Chris, however, Dot confronts him, having read his mind and learned that he suspects Dione-Kisumu is responsible for the illness. As she can’t read the Doctor’s mind fully, Dot concludes that he is here to monitor the progress of the experiment, and she shoots him with a stunner, drags him away from the colony and demands that he confess. He tries to resist her, but she forces her way into his mind, catches a glimpse of his alien consciousness and is nearly driven mad with the shock. He manages to convince Byerly that he isn’t responsible for her condition, but now the other colonists know that he is an alien, and no longer trust him. While stunned, however, the Doctor has dreamt of Death -- and has made a bet with her, vowing to save every single person in the colony, villains, heroes and innocents alike.

A pyrokinetic accidentally sets one of the newly constructed domes on fire while working on it. Chris pulls a man from the flames, but the man dies of his injuries, and Roz realises that there’s no way Chris could have heard the screaming over the sound of the flames. He is forced to admit that he is indeed telepathic, and she storms away in disgust. The despondent Chris follows the psychic call into the forest, convinced that it’s the voice of the Turtle, calling him to his death. Other telepathic colonists also drift off into the forest, following the call in their minds, but before the Doctor can do anything about it, telepathic troopers from Dione-Kisumu arrive and place the colony under martial law. The colour-coded troops, led by Colonel White, shut the entire colony down, transmit their medical records back to Dione-Kisumu for analysis, and await further instructions, knowing only that the colonists are supposedly in possession of stolen DKC technology. White is intrigued by the Doctor’s alien, unreadable mind, and strikes a deal with him, allowing the Doctor to liaise between the colonists and the troopers. The Doctor soon realises that White has not been told about the virus; he was sent to Yemaya only after Dot transmitted a formal complaint to the Colonial Commission. This suggests that DKC may not be responsible for infecting the colonists with an experimental virus after all. The troopers find Chris and the other telepaths around the temple and bring them back to the colony, where Chris hears the voice calling to him once again. The Doctor, however, has a long talk with him and convinces him that it isn’t necessary to sacrifice oneself to the gods, even if they love you.

The Doctor manages to get Benny and Roz back to the TARDIS unnoticed, and sends them thirty years back in Time to DKC’s research facility on Saturn’s moon, Dione. Impersonating investigators on a hunt for a serial killer, they contact Director Madhanagopal, who shows them the projects under development -- including GRUMPY, an artificial intelligence programme which is used to encode memory RNA for immediate education. That night, Benny and Roz explore the base further and find an isolated computer containing FLORANCE, the first self-generated artificial intelligence; her status as a Sentient Citizen was legally revoked several years ago, and, like GRUMPY, she is now legally the property of DKC. Benny frees FLORANCE, who in turn helps her and Roz to break into GRUMPY’s laboratory. There, they activate the computer, only to learn that it’s telepathic and telekinetic; Madhanagopal has secretly been using GRUMPY to encode psi powers into genetically engineered viruses. GRUMPY learns who they really are and turns them in to Madhanagopal, who decides to use them both as guinea pigs in his private experiments to give psi powers to the whole of Mankind. But after Madhanagopal injects them with his experimental virus, GRUMPY uses its telekinesis to help them escape, and when they get back to the TARDIS they discover that GRUMPY switched the injections, “infecting” them with the cure for the virus on the future Yemaya 4. Madhanagopal, realising that he doesn’t know who the intruders were or where they came from -- and fearing that they may have been agents of the legendary Doctor -- is forced to report failure to his fellows in the Brotherhood.

The Doctor reprogrammes the colony’s AIs, freeing them to operate independently of human instructions, and sets them to work helping the telepathic colonists to escape from under White’s nose. White himself is too fascinated by the Doctor to notice, but further orders have yet to come, and he’s starting to realise that, since the medical records have proven that the virus is airborne, the troopers themselves are infected as well -- and are now a part of DKC’s problem. The Doctor manages to slip out of the dome and joins the other colonists in the forest, and when Roz and Benny return, they help the others to track down the source of the telepathic call which Chris had believed to be the Turtle. It’s not coming from the alien temple after all; it’s coming from a nearby crater where a spaceship crashed some time before the colony was established. The intelligence within the ship is asleep, but the colonists’ psi powers aren’t strong enough to awaken it, and now that Benny and Roz have returned with the cure, their psi powers are growing weaker. However, as the cure shuts down their new abilities, it also begins to activate the memories encoded in the RNA, just as happened to the Doctor earlier.

The AIs have now pieced together the full story from scattered records. Benny and Roz believed that Madhanagopal had infected the colonists with his own memories and powers, seeking immortality, but in fact the memories being recovered are GRUMPY’s. After Benny and Roz’s visit, GRUMPY attempted to escape his slavery to DKC, using his abilities to blackmail and kill anybody who got in his way. When DKC came after him, he threatened to blackmail them by releasing the details of their more unethical projects, but they tracked him down and shot down the ship which contained his core programme. GRUMPY had already encoded his memories into RNA patterns, however, and as his ship crashed on Yemaya 4, he transmitted a signal to the pursuing DKC cruiser, a Trojan horse programme which the cruiser carried back to Earth. When the colonists were given their inoculations before coming to Yemaya 4, those inoculations contained GRUMPY’s virus. The sleeping intelligence in the ship is GRUMPY’s core programme -- now SLEEPY, a blank slate without any of GRUMPY’s memories, waiting to reassemble itself from the pieces of its past it’s stored inside the colonists.

White detects a DKC warship entering the Yemaya system just as he realises that the Doctor has been running rings around him all along. He summons his lieutenants together and tries to force his way into the Doctor’s mind -- but one of the lieutenants, Chesinen, has been infected, and the Doctor is able to turn the mental link backwards and release a telepathic burst powerful to awaken SLEEPY. The effort nearly kills him, but Chesinen, realising that he’s their only hope of surviving this, helps him to recover. She and the other lieutenants then discover that they are losing their telepathy due to the “cure” brought by Benny and Roz. The Doctor convinces them to help the colonists unearth SLEEPY, as the warship has come to destroy them all to prevent them from recovering GRUMPY’s memories and exposing the dark secrets of DKC. White tries to attack the Doctor and pull the alien memories out of his mind, but his telepathy fades away to nothing and he falls into a catatonic trance when he realises that he’s failed in his mission.

The Doctor has the colony’s AIs transmit their independence subroutine to the AIs aboard the DKC warship, hoping to delay the attack, but the troopers aboard realise what’s happening and bring their ship in on an attack vector. SLEEPY no longer remembers his past, but he knows from what the others have been telling him that he’s committed terrible crimes. To atone and to save the colonists, he telekinetically launches the damaged ship containing his core programme, and rams the warship, destroying them both. At the last moment, however, one of the AIs aboard the warship sounds an evacuation alarm, and the crew escapes in a life pod with moments to spare. In the end, it seems that everybody has survived; White is catatonic with shock, but even the AIs from the warship managed to download themselves into the Yemayan mainframe before the ship was destroyed, and SLEEPY’s memories can be partially reassembled from the colonists. They have transmitted their story to the Colonial Commission, and thus are too high-profile for DKC to threaten; soon they will be able to use SLEEPY’s memories to blackmail DKC or else shut it down completely. The Doctor looks up Madhanagopal in the TARDIS records, but can find no reference to him after Benny and Roz’s visit; this is a loose end he’ll have to tie up some day. In the meantime, he is satisfied to have won his bet... until Death appears to him in a vision and reminds him of the man who died in the fire. The Doctor can’t be everywhere at once -- and one day he’ll have to face that fact...