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