"The Earth could be at centre of a war that doesn't even belong to this dimension! "

The Doctor and Ace receive a mysterious distress signal and land in the Earth village of Carbury. where a neclear convoy is halted. The Doctor's concern is heightened when knights from another dimension begin to land in the area.
Nearby is Lake Voortigen, the mythical resting place of Excalliber, King Arthur's sword.

But the lake holds a more sinister secret. A spaceship from the same alien dimension lurkes under the lake, but is it really Arthur's corpse? And what is it the witch queen Morgaine, is really after?

The Doctor's old friend the Brigadier is sent to investigate, but the Doctor's happiness is short lived. It seems his friend must take on the might of the Destroyer, a horned Demon intent on bringing about the end of the world...