The Doctor learns through UNIT that a mysterious seed pod has been found deep in the permafrost in Antarctica. When he arrives, the thawed pod has germinated and burst, attacking one of the three men at the small Antarctic base. The man starts to change into a Krynoid, a carnivorous form of plant life which destroys the animal life on every planet where it takes hold. The Doctor finds a second pod which is stolen by agents for an eccentric plant collector. The two remaining scientists are killed, the base destroyed and the Doctor and Sarah are literally out in the cold. Back in London, the collector germinates the second pod. The Krynoid grows the size of a house and demands that the Doctor be turned over to it. Even with UNIT on the job, the odds are difficult-every plant for miles around has now turned murderous, and weapons can't even stop them.