Why is Doctor Who suddenly summoned to the shores of Loch Ness? Terror and panic spread as the third oil rig is smashed into the sea by a mysterious force... the monster?

The controlling power must be the Zygons — alien creatures who have lived hidden on Earth for thousands of years, and now feel strong enough to take over the planet... The Doctor, Sarah and UNIT have different ideas — but can they outwit the supreme cunning of the ruthless Zygons?