The Antarctic, the near future. A strange seed-pod is unearthed from 20-thousand-year-old ice. The Doctor and Sarah go to investigate, but are too late to prevent the pod infecting a scientist and unleashing a threat to all animal life on Earth...

Two employees of the millionaire Harrison Chase arrive at the Antarctic base, and they will stop at nothing to get hold of the second seed-pod.

The Doctor and Sarah Jane return to England after their Antarctic ordeal and Chase tries to eliminate them, but they escape and track him down at his estate, where he is preparing to open the Krynoid pod.

The Doctor is captured by Chase, who plans to breed the stolen Krynoid pod. He is placed in an automated compost hopper and only Sarah Jane can save him... but will she make it before the Krynoid grows to fruition ?

The Krynoid grows to a monstrous size and traps the Doctor and the others in a cottage. The creature demands they turn over the Doctor, while Chase seeks to make an alliance with the alien vegetation.

The Doctor, Sarah Jane, and Scorby are trapped in the manor as a possessed Chase stalks them from within, while the gigantic Krynoid prepares to crush them from without. Worse, the alien plant is ready to germinate, spreading its seeds across England.