As UNIT are transporting a nuclear warhead around Carbury, knights in armour arrive to continue waging a war that began thousands of years ago, but in a wholly different dimension.

The Doctor and Ace discover a stone spaceship underneath Warmsley's archaeological dig while Mordred summons Morgaine to Earth's dimension.

Morgaine forces Ace and Shou Yuing to hand over Excalibur by threatening to release the Destroyer, so the Doctor and the Brigadier enter a dimensional portal to stop her.