Parts: 4
Writer: Robert Banks Stewart
Airdates: 30 August 1975 - 20 September 1975
Production Code: 4F

Doctor: fourth
Companions: Sarah, Harry

Creature Feature: Zygons
Special Guests: UNIT (the Brigadier, Benton)

Story: The Brigadier summons the Doctor, Sarah and Harry to Scotland for aid - an oil rig on the coast has been destroyed, and UNIT suspect the involvement of something out of the ordinary. The Doctor initially scoffs at the idea, thinking the Brigadier's summons inconsequential, but when a second rig is destroyed, his opinion changes. In the wreckage, the Doctor finds what he thinks is a tooth mark - and when he makes a cast of the impression, the resulting tooth is incredibly large.

Harry, investigating a lead, comes across a man who has swum ashore from the most recently destroyed rig - but before he can help the man to safety, a gunman wounds Harry and kills the other man. When he comes to in the hospital, Sarah learns part of his story...and after she leaves the room to telephone the Doctor, something attacks Harry, then goes after Sarah as well.

Harry is taken prisoner by the Zygons, aliens whose planet was destroyed and who now want to terraform the Earth using human slaves. The Zygons have the ability to take on the form of humans they have captured, so the Doctor and Sarah must be wary of who they trust. And as if the Zygons were not enough, they must also deal with the creatures' vicious pet, lurking beneath the waters of Loch Ness...