A strange pod is found in the Antarctic by an expedition party. The pod germinates and attacks one of the people. U.N.I.T. are called in to investigate, and the Doctor and Sarah travel to Antarctica. Also travelling to Antarctica are some of Harrison Chase's men intent on stealing the pod.
The Doctor identifies the pod as a Krynoid pod, and tells everyone they always travel in pairs. Back in the permafrost the Doctor discovers a second pod. Chases's men steal the pod, and destroy the base, believing the Doctor, Sarah and remaining scientists to be inside.
Back in England the Doctor traces the pod to Chase, and finds that he is a man obsessed with plants who believes that animals should give way. Chase takes Sarah's arm in the hope that she will be infected by the Krynoid, although the Doctor intervenes, and one of Chases's men, Keeler, becomes infected.
Rather than help Chase stands by and waits for Keeler to become a Krynoid, which soon happens. The Krynoid then starts to influence other plants, and people within a 1 mile radius start to die in accidents involving plants. Major Beresford orders the RAF to bomb the Krynoid, completely destroying it.
