In Antarctica, British scientists Charles Winlett and Derek Moberley discover a pod buried in the permafrost, and take it back to their camp. John Stevenson, the base botanist identifies it as vegetable-based and estimates it has been buried in the ice for some twenty thousand years.
Back in London, Richard Dunbar of the World Ecology Bureau shows the Doctor photographs of the pod. Although he feels that the Doctor cannot help them, his superior Sir Colin Thackeray insisted. The Doctor examines the pictures and believes it to be extraterrestrial. He tells Dunbar to contact the expedition by their regular video link, and tell them not to touch it until he arrives.
Back at the base, Stevenson discovers that the pod is growing larger and he believes it is absorbing ultraviolet radiation. In England, Dunbar visits the estate of millionaire Harrison Chase. Chase's estate is filled with thousands of plants, and he considers it his mission to protect the plant life of Mother Earth. Dunbar has come to show him pictures of the pod, its possible extraterrestrial origin and hints that such a valuable specimen could easily disappear.... for a price. Dunbar gives Chase the location of the pod. Chase calls for one of his men, Scorby, telling him to take Keeler along.
At the base, Winlett is half asleep near the pod when it opens up. A frond-like tentacle whips out and stings his arm, causing Winlett to collapse in pain. When Stevenson and Moberley find him, Winlett's face is covered with green hives. The Doctor and Sarah arrive at the base. In the sickbay, Winlett's body temperature and pulse are dropping rapidly. His face and body are now covered with a green fungus, and its growth is accelerating. The Doctor asks for a blood test on Winlett, who is growing increasingly monstrous, and examines the now-empty pod. Stevenson acknowledges that it may be his fault; convinced that the pod was alive, he placed it under a lamp. Outside the base, the Doctor digs at the ice, uncovering another pod. Noting that the pods travel in pairs, the Doctors transfers it to the base freezer. On analysis, Winlett's blood is found to contain no blood platelets, but instead has schizophytes — microscopic organisms akin to plant bacteria.