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.