Responding to a distress signal, the Doctor and Ace arrive near the village of Carbury, where a nuclear missile convoy has run into difficulties. The peace of the English countryside is shattered as UNIT troops are beseiged by armoured knights from another dimension. As the crisis deepens, Brigadier Lethbridge-Stewart is called out of retirement to join the battle. Together, he and the Doctor must face the destroyer, a fearsome demon summoned by the witch queen Morgaine to devour the world....