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 whilst the Doctor falls into a trap and is hunted by the giant Skarasen. Meanwhile Harry is held prisoner on the Zygon spaceship.
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.
Broton the Zygon warlord, disguised as the Duke of Forgill, intends to use the Skarasen to attack the World Energy Conference in London.