Trying to take his mind off Susan’s departure, the Doctor decides to accept an old invitation to the funeral of the Venusian philosopher Dharkig, and thus pilots the TARDIS to the city of Bikugih, three billion years in Ian and Barbara’s past, when the Universe was smaller and easier to navigate through. Ian and Barbara are startled to find themselves on an entirely alien world populated by giant reptile-like creatures with five legs each, but the Doctor assures them that the Venusians are perfectly friendly, and introduces them to Dharkig’s daughter Trikhobu. Ian remains wary, and although the Doctor agrees to participate in the remembering of Dharkig, Ian turns down the food which is offered to him. Barbara takes some to be polite, and discovers too late that the “food” was in fact a part of the dead Dharkig’s brain -- and while the Venusians who consume the brain matter merely acquire new memories, the dose overwhelms Barbara’s human metabolism and she begins to relive the memories she has consumed.

Ian, unsure what is happening to his friends, is kidnapped at gunpoint by Havteg of the Rocketeer faction, who demands that Ian teach his people how to build rockets with which to travel to the third planet of the solar system. Ian, still not fully aware how far back he has gone in time, concludes that the Doctor and Barbara have been drugged by hostile aliens who wish to conquer the Earth. Meanwhile, Barbara awakens to find that she has assimilated Dharkig’s memories and with them an understanding of Venusian culture. Venus is dying; as the speed of the planet’s rotation slows, the average temperature is increasing, and soon Venus will be unable to support life. Dharkig had urged his people to accept the inevitable with grace and dignity, but many have turned their back on the message of Acceptance, and will do whatever it takes to see their race survive -- even force visiting aliens to help them, whether they want to or not…

While discussing music and philosophy with his friend Jilet Mrak-ecado, the Doctor is attacked and kidnapped by Jignivi, a member of a faction which believes that it is possible to migrate to cool tunnels beneath the surface of the Sun. As she tries to force the Doctor to assist them, they are confronted by another Venusian who wants the Doctor to help his people grow gills so they can live beneath the oceans. As the two Venusians argue, the Doctor escapes with the help of a passing child named Vivojkhil, but they are caught by Havteg, who threatens to kill Ian unless the Doctor accompanies him. By now, Mrak-ecado has alerted Presidor Jofghil’s guards, who show up and arrest Havteg -- but Jofghil is an incompetent administrator who orders his men to raid the Rocketeer compound, a rash action which could result in Ian’s death.

Ian discovers that free metal is poisonous to Venusians, and that the Rocketeers have therefore been building their prototypes out of wood, with disastrous results. He tries to discourage them from their plan to colonise Earth, but as he speaks with them he comes to realise the extent of their plight and their desperation to escape from their dying world. Jofghil’s soldiers attack the compound, but the Rocketeers force Ian into a wooden crate and evacuate through a secret tunnel, blowing up the compound behind them to discourage pursuit. Ian eventually breaks out of the crate, to find that he has been loaded onto a globeroller heading for the neighbouring country of Inharihib. The crew of the globeroller has never seen anything like Ian before, and in order to avoid charges of kidnapping, the Rocketeers claim that Ian is an experimental animal being transported to a laboratory in Inarihib. This backfires when the crew decide that the escaped animal poses a threat to their passengers and attempt to kill it. Ian escapes, but the globeroller continues on its way, leaving him stranded in a forest of petrol trees -- and as the day draws on, the heat of the sun causes the trees to explode.

The Doctor and Barbara find the remains of the escape tunnel and follow it to the globeroller terminal, where the Doctor sends a heliograph message to Jofghil and Mrak-ecado. Jofghil sends word to the authorities in Inarihib, and then tries to place the terminal co-ordinator under arrest for treason. As Barbara objects, a spacecraft the size of a mountain range suddenly materialises above the city, and the new arrivals, the Sou(ou)shi, claim that they are here to evacuate the Venusians from their dying world and move them to the viable third planet of the solar system. The suspicious Doctor questions their motives and determines to learn more, and Barbara, realising that he has abandoned the search for Ian in favour of this new puzzle, furiously enlists Trikhobu’s aid to rescue Ian herself. The Doctor convinces the reluctant Jofghil and Mrak-ecado to let him accompany them to the Sou(ou)shi ship, where he notes the aliens’ deliberate manner of speech -- as if they are selecting their words with great care to avoid certain truths. Jofghil, believing that he will be regarded as the saviour of his people if he successfully negotiates with the Sou(ou)shi, refuses to listen to the Doctor’s warnings to take care. The Doctor therefore chooses to remain on the ship to learn more, while Jofghil and Mrak-ecado return to Bikugih to organise the evacuation.

Ian is rescued by Jellenhut, a native of a small village near the petrol forests, and as he recovers and watches her children at play he finally comes to understand that the Venusians are not alien monsters, but people. But just as he’s starting to become comfortable, a government death squad arrives in the village with orders to cull the excess population of their country so the ruling classes can survive in comfort. Barbara and Trikhobu arrive in search of help when their globeroller is becalmed nearby, and find Ian and the villagers fighting off the death squad. Upon witnessing the slaughter, Barbara is temporarily overwhelmed by Dharkig’s memories, and, enraged, she joins in the fight, killing one of the squad and driving the others away. Ian is taken aback by the change in her, but she assures him that it is temporary. Jellenhut’s children have been slaughtered, but Ian convinces the grieving Jellenhut not to kill herself; he needs her help to survive, and she is the only Venusian to have shown him kindness since he arrived. In return, when Barbara assures him that it is safe to do so, he agrees to eat part of her child’s brain to remember him. While Ian is overwhelmed by the remembering, however, the bud-brother of the Venusian whom Barbara had killed returns, seeking revenge. Barbara is seriously injured before Jellenhut can kill him, and she must be returned to Bikugih at once for medical treatment.

The Venusians known as the Volcano People are concerned about the arrival of the Sou(ou)shi, and their leader, Mrodtikhil, sends Ruribeg and Nefkhil to consult the seer Kontojij. Kontojij has woken with a sense of foreboding, and has used the most powerful of his predicting crystals to forecast the future -- an all-too-imminent future in which his entire species will be destroyed. Kontojij places copies of the message in the pouch-pockets of his homing gifghoni and sends them all to Bikugih, hoping that at least one of them will survive the scorching heat of the day and convey the message to Mrak-ecado. As he waits for a response, however, a psychic force manifests itself through his crystals, scythes through his home and tears him to pieces. The same force descends upon Ruribeg and Nefkhil when they arrive, but Ruribeg escapes alive with a copy of Kontojij’s message. Meanwhile, one of the gifghoni reaches Bikugih before expiring from exhaustion, and the message is delivered to Mrak-ecado -- who has no choice but to warn Jofghil that the Doctor will attempt to destroy the entire Venusian species before the day is out.

On the Sou(ou)shi ship, the Doctor studies a sculpture composed of mm’x crystals, which the spokesbeing Pown(ow)ri claims was made by another species whom they helped to preserve. But the Doctor sees through Pown(ow)ri’s evasions and forces him to admit the truth -- they “saved” the Aveletians by eating them all and using the detritus to fertilise another world, where life is now developing based on the Aveletian genetic code. The Sou(ou)shi are bound by a strict code of honour, and are thus incapable of telling direct lies or taking life before they are given permission to do so; but they are capable of hiding the truth, and they have already received permission to devour the Venusian people. The Doctor attempts to pilot a shuttle to Bikugih to warn Jofghil of the danger, but the Sou(ou)shi -- although taking no direct action against him -- neglect to set the shuttle’s safety buffers. The shuttle thus disintegrates in the atmosphere, but the Doctor uses the multi-dimensional mm’x crystals to transport himself to the planet’s surface. The Sou(ou)shi learn that he has survived, and inform Jofghil that the Doctor intends to prevent them from carrying out their promise to transport the Venusians to the third planet. Jofghil thus orders the Doctor’s execution.

The globeroller taking Barbara back to Bikugih is badly damaged in a storm, and while the crew attempt to repair it, Ian and Jellenhut set off in search of help. Instead, they are captured by the Volcano People and taken to Mrodtikhil, who explains that Kontojij has predicted that aliens will kill the entire Venusian race before the day is out. From the context of the message, the aliens he described appear to be the Sou(ou)shi. The Volcano People have been preparing to trigger a series of volcanic eruptions which will shroud Venus in dust, lowering its temperature and giving the Venusian race another ten thousand years of life; if the Sou(ou)shi prove to be hostile, they can trigger the eruptions immediately, and destroy the Sou(ou)shi ship in the atmosphere. Ian agrees to return to Bikugih to investigate if the Volcano People agree to help Barbara, and the Volcano People thus usher him into an underground transport. However, as it rushes back to the city it runs into a microthin nenetif web strand, which slices the transport in half, kills Ian’s escorts and leaves him stranded in the tunnel.

The soldier Lighibu and her Five attempt to ambush the Doctor as he enters Bikugih, but Mrak-ecado stops them and tries to convince the Doctor to escape while he still can. The Doctor refuses to do so, insisting instead that he be placed on trial in order to acquit himself and reveal the threat posed by the Sou(ou)shi. Mrak-ecado thus reluctantly presents Kontojij’s message to the Night Council, apparently proving that the Doctor intends to kill them all. Although saddened by what he has seen, the Doctor chooses not to contest his old friend’s evidence, and Jofghil thus sentences him to death. Lighibu, however, understands that the Doctor does not blame her for following what she believed to be proper orders, and realises that he is harmless and that Jofghil is acting out of fear and desperation. She and her Five thus disobey orders and help the Doctor to flee to the TARDIS. Meanwhile, the population of Bikugih begins to board the Sou(ou)shi ship…

The globeroller crew complete their repairs and return to Bikugih, but as they approach the city, they are confronted by soldiers who attempt to kill Barbara. Before they can do so, the Sou(ou)shi transport Barbara aboard their ship, where they place her in a biofeedback force field; they will not kill her or actively harm her, but the field will cause her to suffer intense neurological pain until the Doctor gives them permission to take his life. Back on the ground, as Trikhobu tries to find someone to whom to report Barbara’s kidnapping, she learns what has happened to the Doctor and helps Lighibu and her Five to fight off their pursuers. Meanwhile, the Volcano People send Soneghil and Jellenhut to investigate when they fail to receive word of Ian’s arrival, and they rescue Ian from the tunnels and take him the rest of the way to Bikugih. He arrives just in time to see Lighibu sacrifice her life to get the Doctor and Trikhobu into the TARDIS -- and watches in horror as the TARDIS dematerialises, leaving him and Barbara behind.

The Doctor finds to his dismay that, despite the relative ease of navigation this close to the origin of the Universe, he is still unable to find his way back to Venus. However, Trikhobu knows an old Venusian mathematical formula which she can use to calculate the position of any living being in time and space, and by taking the TARDIS back to the beginning of the Universe and then going forward again, the Doctor finds that he can navigate accurately. He thus tracks down Pown(ow)ri on the Sou(ou)shi ship, but refuses to grant permission for the Sou(ou)shi to kill him even when Pown(ow)ri reveals what has been done to Barbara. Furious, Pown(ow)ri pursues the Doctor into the TARDIS, where he finds Trikhobu, whom he can legitimately kill under the agreement. Within the TARDIS’ state of temporal grace, however, his psychic weapons do not work, and Trikhobu literally kicks him out of the TARDIS, fatally injuring him. The Doctor then tracks down Vivojkhil, the child who helped him earlier, and finds her chastising her baby sister Podsighil for panicking and running away after they were separated from their bud-mother while boarding. Podsighil claims to have seen a friendly female alien elsewhere on the ship, and the Doctor allows her to lead him to Barbara, whom he frees.

Ian is brought before Jofghil, to whom he presents his own copy of Kontojij’s prediction -- and Jofghil realises that the copy which Mrak-ecado had shown him was edited, and that the original makes it clear that the Doctor is not responsible for the slaughter of the Venusians. Mrak-ecado, caught out, promises to reveal all to Jofghil and Ian privately, but once they are alone he pulls out a predicting crystal which enables the Sou(ou)shi to focus their psionic energy in corporeal form. The Sou(ou)shi contacted him some time ago, and he gave them permission to devour and remember the Venusians, for fear that his people would otherwise die lost and forgotten. The Sou(ou)shi tear Jofghil apart before he can escape, and then, much to Mrak-ecado’s temporary shock, they turn on him as well. Once they have eaten their fill, they inform the sickened Ian that the Doctor and Barbara are both aboard their ship -- and unless he orders the Volcano People to terminate their experiments, they will set in motion events leading to his friends’ deaths.

The Sou(ou)shi inform the Venusians aboard their ship that the Doctor has boarded without permission and is trying to prevent the Sou(ou)shi from carrying out their promise. The Doctor and Barbara thus find an angry mob waiting to kill them upon their return, but Barbara, suffering from pain and shock, slips automatically into Dharkig’s mode of thought. And Dharkig, a famous and powerful orator, is able to convince his fellow Venusians to look upon the Sou(ou)shi and see them for what they really are. The Venusians realise that they have allowed their desperation for survival to blind them to the truth, and as the Sou(ou)shi turn on them and begin feeding, the Venusians fight for their lives.

Ian finds that Sou(ou)shi killed and ate Soneghil when he refused to send their message for them -- and, realising that he cannot betray an entire race even to save Barbara and the Doctor, he sends a transmission ordering Mrodtikhil to trigger the eruptions immediately. The Volcano People do so, and the Sou(ou)shi on the ship, caught up in the bloodlust of the battle, fail to recognise the danger until it’s too late. The Doctor gets Barbara and the children into the TARDIS before the Sou(ou)shi ship is destroyed, and, using Trikhobu’s formula, returns to Venus for Ian. The now orphaned children find a new bud-mother in the bereaved Jellenhut, and Ian reveals that the eruptions have shrouded Venus in dust, giving the Venusians a new lease on life. The Doctor, Ian and Barbara depart, hoping to use Trikhobu’s formula to find their way back to Earth in 1963. Meanwhile, the discorporate minds of the Sou(ou)shi, sustained briefly by those they fed upon in the ship, drift to the third planet of the solar system, bind themselves to the crystal minerals of that world, and wait to be found by something they can eat…