A human colony on Gadrell Major is attacked and occupied by the Phractons, whose desperate requests for the mineral porizium -- the only cure for a plague amongst their kind -- have been ignored. The Phracton Commander has ordered that no harm is to come to the humans, but before he can take charge of the operation his xenophobic Secondary kills the governor and bombards the rest of the colony, destroying its support and infrastructure. Darius Cheynor’s Earth Control team is sent to fight the Phractons and supply humanitarian aid, but the Phractons infect the Earth ship’s computers with a virus and the humans find themselves fighting a losing battle. Cheynor eventually comes to suspect that Earth Control is hiding vital facts about the operation from him, and when he researches the history of the colony in a quiet moment he comes to an unpleasant conclusion. As soon as the communications satellite is repaired, he contacts the Phractons and arranges to meet with a diplomatic party in the capital city, Banksburgh -- to negotiate the total surrender of the human forces on the planet.

After spending some time in the TARDIS recovering from his recent injuries and Ace’s departure, the Doctor detects traces of a telepathic link stretching through Time, and follows it to 20th-century London. There, a pregnant Indian woman faints when she catches sight of him, and he realises that her unborn child is part of the telepathic trinity he has detected. He asks Benny to follow the woman while he traces her car’s license plate, and although Benny loses her she does meet the woman’s only friend, her cousin Nita. Nita explains that Tilusha has been shunned by her family for marrying outside her religion, and that her husband Phil Tarrant has turned out to be violent and abusive. Nita, Benny and the Doctor arrive at Tilusha’s home just as Tarrant begins to beat Tilusha in a drunken rage -- and her unborn child responds with a psychokinetic blast which lays Tarrant out cold. Within seconds, Tilusha’s pregnancy progresses to the third trimester. The Doctor decides to remain on Earth to observe events, and sends Benny to investigate at the other end of the telepathic link -- the colony on Gadrell Major…

Suzi Palsson, a librarian in Banksburgh, finds the beautiful red-haired Shanstra lying in the ruins of the city streets, and instantly falls under her seductive spell. Shanstra draws strength from Suzi’s dark memories, including the death of her former lover Colm; some time after he dumped her for another woman, Suzi spotted him drowning, and simply walked away, letting him die. Suzi and Shanstra are attacked by three feral youths left to fend for themselves in the ruins of Banksburgh, but Shanstra uses the psychic strength she has drawn from Suzi to kill one of the youths. The young boy, Trinket, flees in terror, but Shanstra allows the girl Livewire to live; Livewire is burning with hatred for the invading Phractons, and Shanstra can draw a great deal of nourishment from that rage. As Shanstra concentrates on Livewire, however, Suzi finds her unconditional love for Shanstra failing, and flees for her life, realising that she had fallen under the spell of an emotional parasite.

As the Doctor and Nita accompany Tilusha to the hospital, the mind within the unborn child tries to latch onto the Doctor’s powerful mind to accelerate its birth, just as Shanstra attempts to make contact with this other part of herself. The psychic turbulence causes the ambulance window to shatter, and the combination of the cracked window, a wet and slippery road, and a poor driver who swerves into the ambulance’s path while trying to avoid it, causes the ambulance to crash. The unconscious Doctor makes contact with the mind of the unborn child, who has been possessed by the Sensopath Kelzen, and learns that Kelzen is one of three; there are also Jirenal and Shanstra, and the Doctor has unwittingly sent Benny to face Shanstra, the most dangerous of the trinity. Tilusha dies giving birth to a boy named Sanjay, who matures to adolescence within seconds and transforms into Kelzen; the Doctor, however, convinces Kelzen to let him take her away from Earth before she causes any more damage.

As Shanstra’s attempt to connect with Kelzen was disrupted by the ambulance crash, she decides to focus her attention on Jirenal, the other of the Sensopath trinity. In the meantime, to recover her strength, Shanstra takes Livewire to the parlay between Cheynor and the Phractons, and focusses Livewire’s hatred into a blast of psychic energy that destroys the Phractons. The Phracton Secondary thus concludes that Cheynor is not to be trusted, and demands that the Swarm retaliate for the murder of the diplomatic party. Meanwhile, Benny arrives and is reunited with Cheynor, whom she met during the battle with the Garvond. Livewire is taken back to Chenyor’s ship to recover, having been discarded by Shanstra -- who has found a much stronger host in the Secondary, and is urging him to attack the humans.

As the TARDIS approaches Gadrell Major, the Doctor tries to explain the alien concepts of morality and responsibility to Kelzen. She is surprised when he does not try to force her to accept his point of view, as Shanstra had always done, and therefore agrees to wait in the Zero Room -- cut off from Shanstra’s influence -- while she makes up her own mind on the subject. In the meantime, she explains to the Doctor that her trinity was exiled from the far future when something went wrong in their bonding; fearing what they might become as a result, the people of their society cast them into the Vortex. Another accident expelled them from the Vortex and scattered them throughout history, or so it seems; the Doctor suspects that this was no accident, but a deliberate act on Shanstra’s part. He arrives on Gadrell Major just as battle breaks out between the humans and Phractons, and with Kelzen’s help, he and telepath Horst Liebniz track down Shanstra in the Banksburgh community centre and distract her, thus enabling the Phracton Commander to regain control of the Swarm when the Secondary is killed in the fighting.

Shanstra retreats to the ruins of the former vice governor’s mansion, accompanied by Phractons who have not shaken off her influence. The Doctor, meanwhile, negotiates a temporary truce between Cheynor and the Phracton Commander while they try to deal with the third party who is stirring up antagonism between them. Cheynor admits that there are no porizium deposits on the planet; Earth Control deliberately lured the Phractons here in order to keep them occupied in a meaningless war far away from the central human colonies. Leaving Liebniz and Benny to deal with Shanstra, the Doctor takes Cheynor, Suzi and the Commander in search of Jirenal, hoping that he will still be able to reason with the third Sensopath before Shanstra gets to him. As he goes, however, Livewire escapes and sets off to avenge herself on Shanstra.

The TARDIS materialises in the far future, at the Pridka Dream Centre, where visitors from all over the civilised galaxy come to heal and explore the full potential of their minds. Jirenal has already brought the Pridka race under his thrall, however, absorbing them into his consciousness and using their bodies as slaves. Apparently unaware of the danger, the Doctor sends Suzi to the Dreamguide to sort out her own conflicted emotions about Colm and Shanstra, while he tries to speak with the centre’s Director. Jirenal has already killed the Director and seized control of the Dreamguide, and instead of finding it a healing influence, Suzi is trapped in a living nightmare. While the Doctor was occupied protecting Kelzen from Shanstra, Shanstra concentrated her efforts on Jirenal, and now the two beings become united as one. The psychic backlash feeds through Kelzen into the TARDIS, killing the Phracton Commander, and Cheynor rushes out to warn the Doctor of the danger. Back on Gadrell Minor, Benny and Liebniz receive a signal from the Doctor and take the human and Phracton soldiers for the final assault on Shanstra.

The Doctor enters the Dreamguide to find that Suzi has overcome her fears and forgiven herself for Colm’s death, but tells her that this is not enough; she must also accept her responsibility for the act and atone for it. He admits that he suspected that Jirenal had already come under Shanstra’s influence, and that he sent Suzi into the Dreamguide because of her own mental link to Shanstra, to see what Jirenal would do when given the opportunity to merge with Shanstra. The Doctor confronts the Sensopath, who explains that, through the Dreamguide, they will consume the minds of every living being in the galaxy, uniting them all into a single consciousness. But, just as the Doctor had hoped, Kelzen has been watching this all, and now realises that Shanstra never intended to unite with her other two aspects, but to subsume them and become the sole dominant aspect of the Sensopath trinity. Kelzen now accepts her responsibility for the acts of the trinity, and uses her connection to distract Shanstra just as Liebniz’s forces break through the Phractons defending her. Livewire shoots Shanstra when her defenses are lowered, and with Shanstra’s death, Kelzen and Jirenal both die as well.

Sanjay, freed of Kelzen’s influence, decides to remain at the Pridka Dream Centre to explore his full potential. The Doctor returns Cheynor and Suzi to Gadrell Major, which will be abandoned and left as a memorial to war; however, he seems sullen and uncommunicative, and when he and Benny depart, she demands an explanation. He is forced to admit that both Suzi and Cheynor are doomed to die in a shuttle explosion set by renegade Phractons who still don’t understand what happened and blame Cheynor for their Commander’s death. Since Cheynor’s death will satisfy the militant Phractons and secure peace between them and the humans, and Suzi’s death will ensure that no trace of Shanstra’s influence remains, the Doctor is powerless to intervene.