The Doctor, Ben and Polly materialize on Apresar IV, where they find giant slugs roaming a deserted city. One such slug consumes the TARDIS, and while hiding from it Ben falls through a panel into the city's control network. There, he is pursued by robot service units who are feeding the cryogenically frozen Apresarians to the slugs' young. A spaceship lands on the planet to investigate the lack of response from the surface; Captain Galloran is returning terraformer Ellis Melthorpe following a survey of the outlying planets. They are attacked by a slug which the nervous, terrified Ellis identifies as a Mollusi, and Ellis flees as the Doctor is consumed by the Mollusi. Polly accompanies Galloran and his crew to planetary control, where they find Ellis and Ben. Ellis accuses Ben and Polly of killing his people, but before Galloran can execute them the Doctor arrives with a vat of larval Mollusi which he found in Ellis' quarters on Galloran's ship. Ellis admits that he was consumed by a Mollusi on his last expedition and gave in to its will in order to survive; upon returning to Apresar IV, he initiated the Apresarian emergency control procedures, placing the entire population of the planet into cryogenic stasis and programming the robot service units to harvest them as food for the Mollusi. The Doctor has used his own willpower to turn the Mollusi which attempted to consume him against its fellows, and he now instigates a security shut-down procedure which deactivates the service units. The Mollusi young break out of the cryogenic chambers and attack them, but Ellis, blaming himself for the deaths, flings himself at a Mollusi which consumes him. The Mollusi then becomes overwhelmed by Ellis' suicidal self-loathing and despair, and turns on the other Mollusi, killing them all.

Susan is listening to beat poetry at a milk bar when a man suddenly stumbles into the bar and explodes into a mass of protoplasm while babbling about King's Cross. She returns to the TARDIS to tell the Doctor, who goes to King's Cross station to investigate and locates a secret entrance to a government installation. There, he is captured by the alien being Raldonn who runs Operation Proteus for the British government. Some time ago his spaceship crashed on Earth, killing his co-pilot, without whom he cannot generate enough psychic power to depart from Earth. The British government "asked" for his help in their weapons research projects, and, concluding that he was on a planet of savages, Raldonn easily twisted the project to his own ends. He intends to mutate a human being into one of his own kind to provide himself with a new co-pilot, but his current serum is unstable and will convert only one out of a million humans. Therefore he had a group of college students brought to the installation on the pretense that they were testing medical vaccines; in fact, he injected them with the serum and released one to carry the toxin to the surface. Soon, everyone in the world will be infected, and at least one of the victims will be close enough to Raldonn's true species to provide him with the psychic energy he needs to escape from Earth. But as the virus spreads, Raldonn's previous failed experiments are driven mad by its effects, and attack and him. The Doctor uses Raldonn's equipment to develop a genetic stabiliser and releases it into King's Cross station. The stabiliser spreads through the Underground system and blankets the city, returning everything to normal. The Doctor and Susan then return to the TARDIS, but the Doctor is unaware that while waiting for him earlier, Susan was removed and then returned by someone -- or something...

The First Doctor, Steven and Vicki materialize in a computer-city named Xenith, which is delighted to see visitors and sends a translation drone to greet them. But the approaching robot startles Vicki, and the frightened travellers rapidly retreat to the TARDIS and depart, leaving Xenith alone again. The Seventh Doctor, however, returns to Xenith with Benny, to make amends for his previous incarnation's mistake. Xenith reveals that he is dying, and Benny convinces the Doctor to trust to the city's good intentions. The Doctor repairs the damage, restoring Xenith to full life again, and making a new friend.

Taking a break from their travels, the time travellers find themselves on the planet Seetar. The Doctor and Ian talk as they wander along the moonlit beach and soon find themselves surrounded by savages. A giant worm-like creature emerges from the sand and the savages fall to their knees in worship. Ian first distracts the creature with a torch then blinds it by throwing a cricket ball at its eyestalk. As the creature retreats back into the sand, the savages begin to worship the time travellers instead. Ian and the Doctor are offered fruits and made the centrepiece of a great celebration that ends when the savages prepare to sacrifice a young girl. Ian stops them and he and the Doctor return to the TARDIS where Barbara meets them. They leave the planet unaware of what they have done. Years later, the seventh Doctor returns to the planet to stop a second sacrifice being made to giant rock carvings of his former self and his former travelling companion.