The Doctor, Sarah and Harry return to Earth in response to an emergency space-time telegraph from the Brigadier, who asks them to investigate the destruction of a series of North Sea oil rigs.
The Doctor and Sarah answer a distress call and find themselves on Zeta Minor, the last planet of the known universe, where a Morestran expedition has gone missing.
The Doctor realises that Zeta Minor lies on the boundary between the universes of matter and anti-matter and something on the planet won't let the Morestrans leave.
The Doctor tries to convince Sorenson to sacrifice himself for the rest of the crew but Salamar's attempt to deal with the situation causes their problem to multiply.
The Doctor, Sarah and Laurence try to find a way to sever Sutekh's link to the mummies while poacher Ernie Clements finds the priory surrounded by a forcefield.
The Doctor is captured by Styggron and left to die in the destruction of the fake village but Sarah escapes from the disorientation chamber to try and rescue him.
The Tardis lands in a spaceship graveyard on the bleak planet Karn, where The Doctor and Sarah find a evil scientist called Doctor Solon is constructing a new body for the brain of an evil Time Lord known as Morbius, who The Doctor thought was executed by the High Council on Gallifrey. Morbius plots to rule the galaxy with his new body.
The Doctor attempts to restore the Sacred Flame to prove his good intentions to the Sisterhood, while Solon prepares to connect the brain of Morbius to his body.
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.
With a killer Krynoid on the loose, the Doctor, Sarah and the last surviving member of the expedition concentrate on hunting it down, unmindful of Chase's men who, once they learn of the second Krynoid pod (which always travel through space in pairs), work to obtain it and eliminate all possible witnesses.
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.
Sarah is saved from a Krynoid fate and runs off, but the same can't be said for one of Chase's men. Not only is he simultaneously devoured and transformed, his fascinated employer will do all he can to encourage the process. The Doctor gets introduced to Chase's grinder and compost acceleration chamber, which promises him a painful yet swift death en route to making him one with nature, via the garden's nutrient pump. The situation has finally gone too far for Dunbar, who outs his own involvement in this matter and stands up to Chase, only to come face to face with a fully developed Krynoid.
In the interest in self-preservation, Chase's men side with The Doctor against the Krynoid, but the Krynoid - now bigger than the Chase Mansion - invites The Doctor to join it, promising to leave the others alone if he does. People within a mile of the Chase Estate, however, start getting strangled by their plants, while Chase, snapping photos of the Krynoid, has a significant one-on-one encounter with it.
Chase is obviously under Krynoid control. The Doctor suspects possession but, with the mountainous Krynoid nearing maturity and whacking the mansion to pieces, he concerns himself with the bigger picture and orders the mansion - with Sarah and him still inside - to be bombed.
