Parts: 4
Writer: Ben Aaronovitch
Airdates: 6 September 1989 - 27 September 1989
Production Code: 7N

Doctor: seventh
Companion: Ace

Megalomaniac of the Week: Morgaine
Creature Feature: the Destroyer
Special Guests: UNIT (the Brigadier)

Story: Tracking down an interdimensional distress call, the Doctor and Ace arrive in England a few years into Ace's future. The nearby Lake Vortigern is a hub of activity, from an archaeological dig that has so far produced only an ancient scabbard to a convoy of UNIT forces. The Doctor and Ace get a lift from the man in charge of the dig, and once at the lake, they meet Brigadier Winifred Bambera, who is reluctant to trust them.

Meanwhile, at home with Doris, Brigadier Lethbridge-Stewart receives a call from Geneva. When he learns that the Doctor is back on Earth, he decides to come out of retirement and pay Lake Vortigern a visit. And the Brigadier isn't the only person heading for the lake. One by one, other-dimensional forces are massing in the area. They are dressed as medieval knights, but fight with energy weapons and grenades as well as swords. One of the knights, Ancelyn, recognizes the Doctor as Merlin.

What has brought the knights to Earth? Who are they fighting for? How is the scabbard hanging in the inn important? And what secrets does Lake Vortigern hide? As the Earth becomes the battlefield for a war from another dimension, the Doctor, Ace and the Brigadier must uncover the answers and combat the mighty sorceress Morgaine.