The Doctor, Sarah and Harry return to Earth in response to the Brigadier's summons. UNIT are investigating a series of attacks on North Sea oil rigs and have set up a temporary HQ in the Scottish village of Tullock. The attacks are the work of a huge cyborg, the Skarasen, controlled by a group of aliens called Zygons whose spaceship lies at the bottom of Loch Ness. The Zygons plan to take over the Earth as a substitute for their own planet, which has been devastated by solar flares. They are using their shape-shifting abilities to take on the identities of locals whose inert bodies are held aboard their ship.