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.

The Brigadier rescues the Doctor from the stone spaceship before they set out to locate Morgaine before she gains Excalibur.