Parts: 6
Writer: Robert Banks Stewart
Airdates: 31 January 1976 - 6 March 1976
Production Code: 4L

Doctor: fourth
Companion: Sarah

Creature Feature: Krynoid
Special Guests: UNIT

Story: A scientific research team in Antarctica uncovers a pod in the ice. Its depth in the permafrost suggests that it may be 20,000 to 30,000 years old, and the team hopes that it will challenge previously-held assumptions about the origins of plant life on Earth. But when the Doctor sees pictures of the pod, he immediately suspects something is wrong, and takes a trip to Antarctica with Sarah.

Another person is interested in the pod - amateur botanist, collector, horticulture enthusiast and millionaire Harrison Chase. Willing to do anything to obtain the pod for his collection, the ruthless man sends two of his henchmen to Antarctica as well. But before they can arrive to claim their prize, the scientists begin experimentally thawing the pod - and to their astonishment, it hatches, and the tendril contained within infects one of their number.

As the man rapidly mutates into a plant, the Doctor and Sarah find themselves facing a deadly Krynoid, a botanical lifeform from outer space. When a second pod, discovered by the Doctor, is taken back to England, he soon realizes that their problems are only just beginning. For if the Krynoid reaches full growth and gestates, all the plant life on Earth will eradicate animal life forever...