Compassion is oddly affected when interference pulses in the Time Vortex draw the TARDIS to the planet Drebnar, an agricultural colony founded and exploited by the Frontier Worlds Corporation. Compassion and Fitz go undercover as data entry clerks to investigate, but Fitz is promoted to the position of lab assistant to the company’s head of scientific research, Griz Ellis. The Doctor, meanwhile, gets a job with the intelligence division of Reddenblak Corporation, Frontier Worlds’ chief rival, and learns that Dewfurth, one of Frontier Worlds’ founders, has offered to sell information on the top-secret Darkling project to Reddenblak. It soon becomes clear that the interference in the Time Vortex is merely a side-effect caused by the operation of Frontier Worlds’ weather control balloon, but the Doctor has now become intrigued in Darkling, and with the aid of Shaz Mozarno -- another founder of Frontier Worlds, who is suffering from an Alzheimer’s-like condition -- he makes an appointment with Dewfurth to discuss the project further. Dewfurth, however, proves to be suicidally depressed, and before the Doctor can stop him he leaps from the roof of the Frontier Worlds building. The Doctor is pursued by security guards who think he pushed Dewfurth to his death, and although Fitz helps him to escape, he is shot in the shoulder while fleeing over a frozen lake where piranha-like fish use their body heat to try to melt through the ice and reach him. He evades the guards and hides in a cave in the forest, where he falls into a healing coma -- but Temm Sempiter, the CEO of Frontier Worlds, has acquired a blood sample from the trail, and finds the results of the DNA analysis more than a little intriguing...
Frontier Worlds is in the business of genetic crop production, and Fitz’s new position entails analysing stool samples from potential new employees to ensure they are not carrying communicable diseases which could contaminate Frontier Worlds’ fragile biostock. To Compassion’s disgust, he seems more interested in his developing relationship with the beautiful Alura than in investigating Frontier Worlds, and all he has learned is that, despite repeated takeover attempts by Reddenblak, Sempiter has no intention of selling out to them. Alura gets a better job with Frontier Worlds and suggests to Fitz that they move in together, and he realizes that the relationship is more serious than he’d intended and that he will be unable to end it without hurting her. Compassion, meanwhile, uses her earpiece to access Frontier Worlds’ secure databases, and discovers that the Darkling project has something to do with genetic manipulation -- and that Sempiter has acquired a sample of the Doctor’s DNA.
Deep in his coma, the Doctor dreams of Compassion flinging herself into the depths of the Time Vortex, and of dancing with his oldest companion -- while knowing that the dance must end soon. He awakens and emerges into the forest to find the grief-stricken Dewfurth, who has survived his fall, shed his skin and become younger in the process. Dewfurth explains that after founding Frontier Worlds he and his fellows realized that they would not live to see the fruits of their labours, and thus diverted research funds into life-prolongation experiments. The experiments bore fruit with the aid of DNA samples from a form of giant alien plant life -- or so it seemed until the founders learned that their personality flaws were becoming accentuated with each regeneration. Shaz Mozarno’s absent-mindedness has become a complete loss of short-term memory, and Dewfurth’s mild pessimism has become suicidal depression. Furthermore, a second, identical plant recently crashed on Drebnar -- and survived.
Dewfurth shows the live plant to the Doctor, and he recognizes it as a Raab, a rare spacefaring plant which normally lives in space and lands on asteroids and small moons to spawn. The pulses from the weather balloon must have drawn both Raab to Drebnar, and although the shock killed the first and prevented the second from germinating immediately, Frontier Worlds employees have been harvesting genetic material from the second plant -- and will soon expose the seed pods. When this happens, the plant will expel its seeds, which will be unable to escape the planet’s gravity well and will render Drebnar barren -- and Drebnar supplies food to seventy percent of the sector. Dewfurth, unable to handle this revelation, gives the Doctor his identity card and flings himself into a nearby compost shredder before the Doctor can stop him. The Doctor uses the i-card to take the cable car up to the weather balloon, where he sends a coded e-mail to Compassion telling her what he has learned and attempts to shut down the weather control systems to prevent them from luring anything else to Drebnar. Before he can do so he is captured by a security robot, which has been programmed to prevent actions which could cause harm to Frontier Worlds or its holdings, and is handed over to Sempiter.
Compassion receives the Doctor’s e-mail, locates the secure files on the Darkling project and sends them to Fitz. Fitz pickpockets Ellis’ i-card and tries to break into the secure Level 10, but is delayed when Ellis takes the same lift to Sempiter’s office, looking for his missing card. Fitz gets away from Ellis only to run into Sempiter, and after making a weak joke to cover, he heads for the emergency stairs and uses Ellis’ card to get to Level 10. There, however, he is forced to hide when Sempiter arrives, returning to his secure office -- and as he watches, Sempiter’s dead outer skin splits open to reveal a new, raw, younger body beneath. Fitz retreats in shock and runs into Ellis, and in order to cover himself he hands over Ellis’ i-card, claiming to have found it while helping the ill Sempiter into his office. Compassion, meanwhile, finds that although the actual location of the Darkling project is classified, the computer systems have automatically assigned it a postal code, thus enabling her to locate it in the grain fields where Frontier Worlds grows its genetically copyrighted crops. Realizing that, thanks to Fitz, they are close to being exposed, Compassion orders Fitz to dump Alura and meet her on Level 10 to learn the truth -- but Fitz, unable to break up with Alura to her face, instead waits until Alura falls asleep, leaves her a note and sneaks out of his apartment.
Sempiter interrogates the Doctor, who realizes that Sempiter pushed for the experimental treatment with the Raab DNA because he was suffering from a fatal, hereditary nervous disorder -- but his personality flaws have also been accentuated by the process, and he has become utterly ruthless and amoral in his pursuit of genetic perfection and Frontier Worlds’ corporate supremacy. It is to this end that he has authorised the Darkling project, an attempt to splice Raab DNA into ordinary wheat to make the crops grow faster and put Reddenblak out of business through sheer supply. The Doctor tries to warn Sempiter of the danger posed by the Raab, but Sempiter is beyond reason and attempts to vivisect the Doctor to learn more about his astonishingly organised DNA structure. The Doctor, cracking witticisims to distract Sempiter, manages to escape and lock Sempiter into his own laboratory, but upon reflection Sempiter realizes that he’s heard one of the Doctor’s jokes before -- from Ellis’ new lab assistant. As soon as he escapes, Sempiter pulls up the employee profiles on Frank and his sister Nancy, and discovers that their DNA is unnaturally organised and efficient, almost as if it had been manufactured.
Fitz and Compassion break into Level 10, where they find the rejected plant-animal hybrids from the Raab experiments. They are nearly captured by the sadistic head of security, Kupteyn, but manage to escape -- but Fitz, fearing that Kupteyn will try to trap him at his apartment, insists upon returning to warn Alura. Compassion reluctantly accompanies him, but they arrive too late and find that Alura has been stabbed to death. Compassion and the numbed Fitz flee as guards arrive, but outside they find the panic-stricken Ellis, who claims that the guards came for him when he learned the truth about the vile experiments on Level 10. Compassion forces Ellis to help them escape in her flitter, but they are brought down by a pursuit craft and crash in the forest. Fitz awakens to find Ellis unconscious, and Compassion in surprisingly uninjured form considering that she had been thrown through the windshield. Compassion tells Fitz to leave Ellis behind, and sets off in the direction of the signals from the Darkling research centre. Fitz accompanies her, despite his growing concerns; he’s been worried for some time about how his lifetimes in the remembrance tanks of Anathema and his journeys in the TARDIS have changed him -- but how have they changed Compassion?
Ellis manages to track down Fitz and Compassion, claiming that they left a trail, but Compassion detects a faint signal coming from Fitz’s arm and realizes that Ellis implanted him with a tracer during his employee orientation. Ellis admits that he detected the oddities in Fitz’s DNA when Fitz was first hired, which is why he promoted Fitz to become his laboratory assistant, but insists that he is on their side. However, he slips up when he mentions something only Alura could have known, proving that he was present when Kupteyn murdered her. Ellis pulls a gun on the furious Fitz, explaining that he’s had enough of Frontier Worlds and wants to sell out to Reddenblak, taking Fitz with him so they can analyse his odd genetic structure. He tries to kill Compassion, but Compassion strikes first, killing him with an axe from the flitter’s emergency supply kit. Ellis begins to shed his skin just as Sempiter and Dewfurth had done, but Compassion chops his body to bits, ensuring he will not regenerate and survive. Fitz is ready to give up, but Compassion talks him out of his despair, convincing him that she needs his humane perspective to complete herself.
Fitz and Compassion continue on to the research fields, where they find that only the genetically engineered Darkling crop can resist Frontier Worlds’ patented weed killer -- and since no other plants can grow in the fields, no animals can feed here, throwing the planetary ecosystem out of kilter. Compassion accidentally sets off a fire alarm while trying to break into the research centre, and attracts the attention of Kupteyn and his security guards. Kupteyn pursues them out into the fields and tries to run them down with a combine harvester, but Fitz manages to circle around, climb aboard and push Kupteyn out into the harvester’s blades. Fitz falls from the harvester before he can change its course, however, and it drives straight into the research centre’s fuel tanks. Fitz and Compassion flee as the exploding fuel tanks set the entire Darkling crop ablaze, but just as Fitz is about to congratulate her and himself on a job well done, the Doctor arrives and informs them that updrafts from the fire will spread the Darkling seeds across the entire planet.
The Doctor has just come from a productive meeting at the Reddenblak Corporation, and he sends Fitz and Compassion back to the city to shut down Level 10 and meet the Reddenblak executives while he uses the weather control system to set things right. He manages to trick the security robot into boarding the cable car and sends it back to the planet’s surface, and then attempts to generate a rainstorm which will wash the Darkling seeds out of the sky and a blizzard which will destroy the live Raab specimen. Unfortunately, he gets the mix wrong when he also attempts to generate sunshine over the city to greet Fitz and Compassion, and the weather control balloon is struck repeatedly by lightning and falls from the sky. Meanwhile, despite Fitz’s moral qualms, Compassion destroys the experimental creatures on Level 10, but both are captured by Sempiter and his guards before they can escape. Sempiter prepares to take them to his laboratory for study, but they are saved by the arrival of Reddenblak executives with shocking news for Sempiter. Ellis’ former lover has sold them the shares he bequeathed to her, and the Doctor has sold them Dewfurth and Mozarno’s shares -- and following the disastrous destruction of the Darkling crop, Frontier Worlds’ common stock has plummeted, enabling Reddenblak to buy the lot. In short, despite all of Sempiter’s attempts to block them, Reddenblak has now become the majority shareholder in Frontier Worlds.
The Doctor arrives, having escaped the destruction of the weather balloon, and tries to convince Sempiter to give in gracefully -- but he will not do so. If he cannot be a part of his own company any longer, he will be a part of the Raab. Sempiter flees, his skin transforming into vegetable matter, and the Doctor, Fitz and Compassion pursue him, realizing that he intends to release the Raab seed pods. By the time they reach the Raab, however, Sempiter has located the security robot, which holds them off while Sempiter prepares to plant a bomb in the Raab’s body and expose the seed pods. The Doctor, however, manages to convince the robot that Sempiter is no longer acting in the interests of Frontier Worlds, and the robot thus turns on him. Sempiter flees out onto the lake of ice, and the pursuing robot cracks through the surface and sinks to the bottom -- but just as Sempiter appears triumphant, the carnivorous fish melt through the surface of the ice and consume him.
Although morally little better than Frontier Worlds, Reddenblak is at least less dangerous. Having chosen the lesser of two evils, the Doctor therefore arranges for them to destroy the Raab using a potent industrial weedkiller, despite knowing that this will also poison the lake and most of the local wildlife. Satisfied that good has been done to the best of his ability, he prepares to depart, but there is still Compassion to deal with -- she claims to have meant none of what she said to Fitz in the woods, and knows that Fitz and the Doctor are trying to teach her humanity. But if she changes at all due to her time in the TARDIS, it will be on her own terms.