The Doctor, Sarah and Harry return to Earth. UNIT is investigating a series of attacks on North Sea oil rigs and has set up a temporary HQ in Scotland. 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. They plan to take over the Earth as a substitute for their own planet. They are using their shape-shifting abilities to take on the identities of locals whose inert bodies are on their ship.