When the Doctor, Sarah Jane and Harry arrive in Scotland, having received an urgent request for assistance from the Brigadier, they discover that the mysterious force which has destroyed three oil rigs has left giant teeth marks on the wreckage. The mystery deepens, leading them to the shores of Loch Ness where they find that the legendary monster really does exist – and is the murderous tool of the Zygons, aliens intent on overpowering the planet. The Doctor, his companions and UNIT must find a way to defeat the deadly Loch Ness Monster and its controllers, but the Zygons have the terrifying power to change shape. The Doctor's life has never been in more danger, as the line between allies and enemies is tested to the very limit...
Tom Baker
as The Doctor
Elisabeth Sladen
as Sarah Jane Smith
Ian Marter
as Harry Sullivan
Nicholas Courtney
as Brigadier Lethbridge-Stewart
John Levene
as RSM Benton
John Woodnutt
as Duke of Forgill / Broton
Keith Ashley
as Zygon
Ronald Gough
as Zygon
Lillias Walker
as Sister Lamont
Angus Lennie
as Angus
Tony Sibbald
as Huckle
Hugh Martin
as Munro
