Returning to Earth, the Doctor, Sarah Jane and Harry arrive in the Scottish Highlands to investigate the mysterious destruction of several oil rigs in the North Sea. Local superstition speaks of a deadly threat that lurks in the mists of Tulloch Moor, but the truth is even more disturbing: the legendary Loch Ness monster is a terrifying cyborg beast which is controlled by the Zygons, an advanced alien race who are desperate to ensure their own survival, at any cost.