Two alien seed pods are found buried in the Antarctic permafrost and the Doctor realises that they are from a Krynoid, a form of plant life that infects and transforms all animal life on planets upon which it becomes established.

One of the pods infects a scientist at an Antarctic base but the developing Krynoid is destroyed by a bomb set by two men, Scorby and Keeler, who have made off with the other pod for their boss, eccentric plant collector Harrison Chase.

At his mansion in England Chase arranges for the pod to be opened under controlled conditions while a human host - Sarah - is held nearby. The Doctor rescues Sarah, but Keeler is infected. Keeler's transformation into a Krynoid is accelerated by Chase, who has him fed with raw meat.

The creature escapes and goes on the rampage, rapidly growing to giant proportions. Chase decides to turn the Doctor into compost by feeding him into a pulverising machine. The Time Lord escapes, but Chase falls into the machine and is killed. UNIT have meanwhile been called in, and they arrange for the Krynoid to be bombed before it can spread its pods across the Earth.