The Doctor, Sarah and Harry return to Earth to help the Brigadier discover what is causing the destruction of oil rigs off the coast of Scotland.
Sarah is attacked by a Zygon disguised as Harry who is a prisoner on the Zygon spaceship. The Doctor falls into a trap and is hunted down by the giant Skarasen.
The Doctor and Sarah find their way into the Zygon spaceship at the bottom of Loch Ness while the Brigadier prepares to force it to the surface with depth charges.
The Zygon warlord, Broton, disguised as the Duke of Forgill, intends to use the Skarasen to attack the World Energy Conference in London.
