A UNIT nuclear convoy, stranded on the shores of Lake Vortigern, becomes the focus of an incursion by knights from a parallel reality. In this other world, technology and magic exist side by side, and the legends of King Arthur are fact. Close to the lake, the sinister Mordred battles against his enemy Ancelyn and summons his mother, the powerful witch Morgaine. Is the Doctor really Merlin? And will he discover what actually happened to King Arthur? But time is running out for everyone as Morgaine takes control of the nuclear weapons and summons the Destroyer – Lord of Darkness and Eater of Worlds…