The TARDIS, responding to a distress signal, materializes near Lake Vortigern in England. There, the Doctor and Ace first hear and then discover Brigadier Winnifred Bambera (Angela Bruce) of UNIT leading a nuclear missile convoy. The retired Brigadier Lethbridge-Stewart (Nicholas Courtney) is told of the Doctor’s arrival and a helicopter brings him from his country home. At the Gore Crow Hotel, they meet a woman named Shou Yuing (Ling Tai), who talks explosives with Ace. Brigadier Bambera stops to examine a blue box at the side of the road and is caught in a crossfire between two armies of armoured knights using swords and futuristic guns.
The Doctor examines a scabbard excavated from a battlefield. It is hot to the touch and the hotel owner’s wife Elizabeth (June Bland), who is blind, says it is waiting for someone. An archaeologist named Warmsly (James Ellis) dates it to the 8th Century, but the Doctor thinks it is much older. A knight, thrown by an explosion, falls through the roof of the brewery. He is named Ancelyn (Marcus Gilbert) and he recognizes the Doctor as Merlin. They are surrounded by knights.
Bambera faces down their leader Mordred (Christopher Bowen), but he is more frightened of the Doctor, who he believes was imprisoned by his mother Morgaine (Jean Marsh). The Doctor threatens him and Mordred’s troops retreat. Mordred begins a ritual and the scabbard flies across the room into his hands. Morgaine arrives through a rift in time and space. The next day, Warmsly shows the Doctor where he found the scabbard. They discover a rune, written by the Doctor himself, that says, “Dig Hole Here.” Ace does that with a cannister of Nitro-9.
Lethbridge-Stewart’s helicopter arrives, but it is shut down by sorcery. As Morgaine’s knights hold a remembrance of Earth’s soldiers, he meets with Morgaine, who warns him she will kill him next time. In the tunnel opened by Nitro-9, they find a chamber under the lake which opens to the Doctor’s voice. He thinks Merlin may be his future self gone back in time. The chamber is part of a spaceship. The body of King Arthur lies next to the sword that summoned everyone. When Ace removes the sword, she releases a glowing entity. Ace hides in an alcove which fills with water. The entity knocks the Doctor out, but the Doctor comes to and ejects Ace from the spaceship. Ace, the Lady of the Lake, rises from the water with the sword Excalibur. The Brigadier arrives, destroys the creature and releases the Doctor.
Mordred and Morgaine come to the hotel for Excalibur. Morgaine takes from the mind of Officer Lavel (Dorota Rae) and turns her body to dust, but then she restores Elizabeth’s sight. UNIT troops are evacuating everyone. The Brigadier shows the Doctor his specialised ammunition, and when the Doctor jokes about silver bullets, rushes off to get some. The Doctor tells Ace to draw a chalk circle around herself and drives off in his old car Bessie. Ace and Yuing draw the circle about themselves and Excalibur. Mordred unleashes the power of the Destroyer. Ace gives up the sword to Mordred, saving her life.
The Doctor and the Brigadier capture Mordred. Returning, the Doctor finds the hotel in ruins, but those in the circle are safe. He finds a portal to Morgaine’s castle and enters with the Brigadier. The Brigadier shoots the Destroyer with no effect, and the return fire sends him flying. Ace rushes in and knocks Excalibur from Morgana’s grasp, though she grabs it again and hurls it through the portal.
Morgaine binds the Destroyer with silver chains, so the Doctor loads the Brigadier’s revolver with silver bullets. The Brigadier knocks out the Doctor and goes to confront the Destroyer himself. He tells the Destroyer to “get off my world” and empties the weapon into the Destroyer’s chest. The Destroyer explodes and the castle is engulfed in flames, but the Brigadier rises unharmed. Back at the convoy, Morgaine and Mordred try to detonate a nuclear missile. She demands to fight King Arthur, but the Doctor reveals that he is dead. Mordred and Morgaine are imprisoned, and the victors gather at the Brigadier’s place.
Lethbridge-Stewart was to die in the early drafts of the script, but the writers thought better of it. It was his first appearance since The Five Doctors. He did not appear again in Doctor Who but had a role in the Sarah Jane Adventures spinoff. The water tank that imprisoned Ace in one scene cracked and water poured over live wires on the studio floor. Thanks to Sylvester McCoy’s quick thinking and timely warning, Sophie Aldred was pulled out of danger in time. His warning was laced with swear-words to show that it was not part of the script. Jean Marsh had been in The Crusade and the Daleks’ Master Plan with William Hartnell, so she appeared with the first and the last of the classic doctors. This was, in fact, the first story of the final season.
Winnifred Bambera, the new Brigadier, was a great action-hero character, the sets were beautiful, and the story was fast-paced, with exciting music. Despite this, the appearance of the beloved Lethbridge-Stewart, the impressive Satanic Destroyer, the romance of the Arthurian legend, and the brilliant Jean Marsh, the story received low ratings. Jean Marsh and script-writer Ben Aronovitch created a fascinating character in Morgaine. At one point, after reducing an attacking UNIT soldier to a pile of dust, she pauses before leaving to cure the innkeeper’s wife of her blindness as payment for her son’s bar-bill. It reminds me of Hercules, who having disturbed his host’s mourning with his drunken carousing, goes down to Hell and brings his wife back again. And the scene in which the Doctor passionately describes the effects of a nuclear explosion to talk Morgaine out of using the cowardly weapon, and then breaks her heart with the news of her lifelong enemy King Arthur’s death a thousand years ago, is really quite memorable.