Members of the World Ecology Bureau discover a centuries-old seed pod buried deep in Antarctica's permafrost. It seems to be still alive, growing without soil. They transmit a photo to London where Richard Dunbar of the W.E.B. shares it with the Doctor. Wary of what it might be, the Doctor immediately flies down with Sarah. They don't know that Dunbar is a leak in the bureau and has tipped off the discovery to a rich plant fanatic named Harrison Chase. Chase sends a thug and a plant expert down to verify the discovery and fetch it back by any means possible. But before anyone arrives, the pod hatches and the tendril-like life form within attaches itself to one of the men, turning him green. It's as The Doctor feared: they're dealing with a Krynoid.

Surviving two attempts to end their lives, the Doctor and Sarah seek the identity of the plant fanatic who ordered the hit on the arctic base, pursuing their only lead - a commissioned painting of a rare flower left in the boot of a car. Chase, meanwhile, is doing all he can to encourage the hatching of the Krynoid pod.