Another adventure for everyone's favourite time-traveller. Answering the Brigadier's space/time telegraph, the Doctor (Tom Baker), Sarah (Elisabeth Sladen) and Harry (Ian Marter) arrive in the village of Tulloch, near Loch Ness. A series of attacks have taken place on local oil rigs, and many are blaming the legendary monster. The Doctor discovers the creature to be the Skarasen - cyborg pet of invading aliens the Zygons. Their own planet having been destroyed and their spaceship crippled, these deadly shape-shifters are now intent upon taking control of the planet Earth.

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.