The Doctor (Colin Baker) is suffering from some post-regenerative trauma, and, after trying to strangle companion Peri, takes off to a deserted asteroid to become a hermit. There he discovers a plot by the Gastropod Mestor to kidnap two mathematically brilliant twins and shift some planets around in order to release more of his slimy ilk. Obviously he has to be stopped, but is the Doctor up to it? Colin Baker's first outing as the Doctor.

An alien pod discovered in the Antarctic takes over one of the scientists who found it. The Doctor (Tom Baker) and Sarah arrive too late; the scientist has already become a Krynoid, a carnivorous alien plant life. Although they manage to destroy the first Krynoid, a second pod is obtained by plant fanaticist Harrison Chase, who intends to nurture the creature to his own ends. The Doctor calls in UNIT to stop Chase before the Krynoid grows and destroys all animal life on the Earth.