DRAWN BY A MYSTERIOUS DISTRESS SIGNAL echoing forwards, backwards and even sideways in time, the TARDIS lands near the English village of Carbury at a point some years into Ace’s future. The Doctor and Ace meet local archaeologist Peter Warmsly and accompany him to his excavation by the shore of the nearby Lake Vortigern. There they encounter a UNIT convoy headed by Brigadier Winifred Bambera, charged with transporting a nuclear missile.
These terrestrial affairs are soon interrupted by the arrival of two factions of armoured knights. Descending from outer space, these visitors come from a parallel world where the Arthurian legends of our universe are closer to reality. Now, both sides are seeking Excalibur, the legendary sword of King Arthur, and swiftly engage in combat. On one side is Ancelyn, a knight loyal to the king, and on the other is an army commanded by Arthur’s sworn enemy, Mordred.