Knights from a parallel universe arrive on Earth to find the legendary sword Excalibur. Only the Doctor and Ace with the assistance of Brigadier Lethbridge Stewart can save the Earth from total catastrophe.
Furious at her inability to engage Arthur, Morgaine threatens to unleash the Destroyer. The Doctor has to escape Arthurs ship under the lake.
The Brigadier takes on the Destroyer while Morgaine takes control of UNIT's warhead..
The Doctor and Ace explore Victorian house Gabriel Chase and realise it hides just as many secrets in it's drawing room as it does in the dark cellar.
The Doctor and Scotland Yard Inspector MacKenzie, attempt to question Josiah Smith, while bot Light and Control emerge from their relative slumbers.
Only Redvers, still searching for the elusive Saxe-Coburg, seems to be able to withstand Light and communicate with Control and Ace realises her past actions need to be dealt with.
Ace and the Doctor arrive in Perivale and Ace becomes cornered while they are trying to discover the cause of people's disappearance.
The Doctor and a number of people from Perivale follow Ace to the Cheetah People's world.

While Ace tries to resist the influence of the Cheetah Planet, the Master tries to leave the planet before it's too late.