Centuries ago, a Zygon spaceship crash landed in Loch Ness. Now, with their home planet destroyed, the alien creatures plan to take over Earth. Their most powerful weapon is a huge armoured dinosaur-like creature of terrifying power that they brought to earth as an embryo – the Loch Ness Monster. The Doctor, Sarah and Harry soon discover that the Zygons have another weapon. They can assume the identity of any human they capture. Who knows which of their friends might really be a Zygon?