Returning to Earth, the Doctor, Sarah 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 on 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...