The TARDIS arrives on present day Earth where the Doctor and Ace investigates 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, where The Doctor is mistaken for Merlin, and the Doctor believes Merlin may be his future incarnation. Reuniting with Brigadeer Lethbridge Stewart, The Doctor, Ace, a young Chinese woman named Shou Yuing, U.N.I.T officer Brigadeer Winifred Bambara and a knight named Ancelyn set out to stop Morganie and her knights and prevent her from unleashing the demonic Destroyer of the Worlds and prevent Morgaine and Mordered from destroying the world with a nuclear warhead, U.N.I.T is transporting across the countryside.