Terror of the Zygons

From http://www.bbc.co.uk/doctorwho/classic/episodeguide

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.

The Doctor releases the Zygons' prisoners and causes their ship, which has now emerged from the Loch, to self-destruct. Only their leader, Broton, survives. He has assumed the identity of the Duke of Forgill and travelled to London. There he plans to give a show of strength by destroying a World Energy Conference with the Skarasen, which is now approaching up the Thames.

Broton is shot by UNIT troops and the Doctor throws the Skarasen its homing device, which it devours. Without a controlling influence, the creature makes its way back to Loch Ness.

3 out of 94 (3.2%) raters say this story requires a previous story.