The Doctor and Ace receive a mysterious distress signal and land in the Earth village of Carbury, where a nuclear convoy has 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 Excalibur, King Arthur's mystical sword...

But the lake holds a more sinister secret. A spaceship from the same alien dimension lurks under the lake - but does it really contain Arthur's corpse? And what is it that the Witch Queen, Morgaine, is really after?

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