Doctor Who: The Fourth Doctor

The hugely powerful Key to Time has been split into six segments, all of which have been disguised and hidden throughout time and space. Now the even more powerful White Guardian wants the Doctor to find the pieces. With the first segment successfully retrieved, the Doctor, Romana and K9 trace the second segment of the Key to the planet Calufrax. But when they arrive at exactly the right point in space, they find themselves on exactly the wrong planet - Zanak.

Why is the Doctor 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 hidden on Earth for thousands of years, and who now feel strong enough to take over the planet. The Doctor, Sarah, Harry and UNIT have different ideas - but can they outwit the supreme cunning of the ruthless Zygons?