The Doctor and Ace respond to a request for help from U.N.I.T., the Earth based organisation who supports The Doctor against the enemies. The location for the help is Lake Vortigern in Cornwall. Lake Vortigern is the mythical location of the resting place of Excalibur, the sword of King Arthur. They have arrived at the scene of a sword battle between two factions which is completely the wrong time.
Along the way, The Doctor seems to unwittingly recruit a new companion, Shou Yuing, a young female who has much the same interest as Ace. The retired Brigadier Lethbridge-Stewart is called back into service to help out with the situation. It soon transpires that Morgan, the arch enemy of King Arthur (as seen in Camelot and Merlin television series) is on the scene with her son Mordred, they are after King Arthur who they wish to kill.
During the episode, the Doctor is reunited with Bessie, the convertible car that appeared most with the Third Doctor. Aiding Morgan is a Destroyer of worlds, a blue devil like creature. The monster can only be killed by silver bullets which the Brigadier obtains and goes after the monster. Morgan gains control of nuclear weapons which she plans to unleash on the planet but the Doctor manages to persuade her not to.
The Brigadier returns to retirement but when he gets there, he finds his wife is off out with Ace, Shui and the young Brigadier Bombara who had earlier been in control of the weapons before Morgan had obtained them.