Doctor Who and the Loch Ness Monster

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?