The TARDIS arrives on present day Earth where the Doctor and Ace investigate a distress signal, only to discover knights from another dimension led by the evil sorceress Morgaine and her son Mordred have arrived in our dimension to seek out Excalibur, the mythical sword of power of the legendary King Arthur. The Doctor is mistaken for Merlin, and the Doctor believes Merlin may be his future incarnation. Reuniting with Brigadier Lethbridge Stewart, The Doctor, Ace, a young Chinese woman named Shou Yuing, U.N.I.T officer Brigadier Winifred Bambara and a knight named Ancelyn set out to stop Morgaine and her knights unleashing the demonic Destroyer of the Worlds and prevent Morgaine and Mordred from destroying the planet with a nuclear warhead that U.N.I.T is transporting across the countryside. Written by
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[sees the Brigadier's body
You stupid, stubborn, pigheaded numbskull! You were supposed to die in bed!
Nonsense, Doctor!... Oh, really, Doctor, do you think I'd be so stupid as to stay inside, do you? Have a little faith!