Knights from a parallel universe arrive on Earth to find the legendary sword Excalibur. And only the Doctor and Ace with the assistance of Brigadier Lethbridge Stewart and save the Earth from total catastrophe.
Did You Know?
This story takes place 'a few years in the future' from Ace's starting point. This is made evident when Ace is surprised that the combined costs of a glass of water and a glass of lemonade are five pounds and when the Brigadier gets a phone call from Geneva and mentions that England now has a king (he cannot be referring to the king of Switserland because the Swiss don't have a monarchy). See more
[the Brigadier knocks out the Doctor
Sorry, Doctor, but I think I'm rather more expendable than you!