In response to a distress signal, the Seventh Doctor and Ace materialize the TARDIS near Lake Vortigern in England. The sound of explosions leads the TARDIS crew into the acquaintance of Brigadier Bambera of UNIT, in charge of a nuclear missile convoy. Following from the encounter, the retired Alistair Lethbridge-Stewart is informed of the Doctor's return, and a helicopter is sent to his country home to collect him, against the protests of his wife, Doris.
Later, at the Gore Crow hotel, the Doctor and Ace meet a young woman called Shou Yuing, who shares Ace's love of explosives. Meanwhile, as Bambera stops to examine a blue police telephone box, she is caught in the crossfire between two groups of armoured knights, using both swords and futuristic guns.
The Doctor shows interest in a scabbard, excavated from the battlefield. The scabbard is hot to the Doctor's touch, and the hotel owner's blind wife, Elizabeth, says she can sense it waiting for something, or someone. When the archaeologist Warmsly arrives at the hotel, he dates the scabbard to the 8th century. The Doctor senses that it has been waiting for far longer.
As Ace and Shou Yuing share a talk about explosives, a knight sails through the brewery's roof, making a huge commotion. On investigation, the Doctor, Ace and Shou Yuing find the knight Ancelyn — who wakes, to recognize the Doctor as "Merlin". While the Doctor mulls the portent of this revelation, the party is surrounded by an ominous group of knights.
Bambera faces down their leader, Mordred. He is shocked to see "Merlin", who he believed bound by his mother, Morgaine. Following some vague threats from the Doctor, Mordred's knights retreat. Later, as Mordred begins an arcane ritual, the scabbard in the hotel flies across the room. Morgaine arrives on the scene through a rift in space and time; she proceeds to psychically taunt the Doctor.
The next day, Warmsly shows the Doctor where he uncovered the scabbard. They find a rune, which the Doctor translates to "Dig hole here." On further question, he replies that it is his own handwriting. Using a canister of Nitro-9, Ace blows an opening.
On arrival in Carbury, Lethbridge-Stewart's helicopter is shot down by Morgaine's sorcery. As Morgaine's knights hold a remembrance ceremony for the soldiers of Earth's world wars, Lethbridge-Stewart has a peaceable encounter with Morgaine — though she threatens Lethbridge-Stewart, in the event they should meet again.
The Doctor and Ace enter a chamber under the lake, finding the door keyed to the Doctor's voice. The Doctor tells Ace that this "Merlin" may well be a future version of himself. Presently, they realize that the chamber is part of an organic spaceship. They also find the body of King Arthur. When Ace removes a sword from a plinth, she activates a defence mechanism, unleashing a hostile, glowing entity. In attempt to hide, Ace enters an alcove. A door closes, and the alcove starts to fill with water. As Ace yells for help, the entity knocks the Doctor unconscious.
The Doctor recovers just in time to fiddle with a control panel, and eject Ace from the space ship. As Ancelyn and Warmsly stand at the shore, discussing the Lady of the Lake, Ace emerges, still grasping the sword. Ancelyn identifies it as Excalibur. The Brigadier arrives on-scene, in time to destroy the creature below the lake and rescue the Doctor.
Mordred and Morgaine go to the hotel, to retrieve Excalibur. When Lavel shoots, Morgaine simply catches her bullet with sorcery. Morgaine takes knowledge from Lavel's mind, then turns her body to dust. As she leaves, she pays Mordred's drinking tab by restoring Elizabeth's sight. Meanwhile, UNIT troops are staging an evacuation. The Brigadier shows off some of UNIT's specialized ammunition, and the Doctor inquires about silver bullets.
The Doctor instructs Ace to draw a chalk circle around herself, to protect against Morgaine's sorcery. He then drives off in his old car, Bessie, hoping to halt a battle between Morgaine's knights and the UNIT soldiers. A storm breaks outside the hotel, so Ace and Shou Yuing draw the circle around themselves and Excalibur. A localized night falls. Within the circle, Ace and Shou Yuing start to bicker. Ace nearly leaves the circle, before they realise they are being toyed with.
Just as Mordred and Ancelyn are about to fight, the Doctor intervenes. Mordred, however, reveals that this battle is merely a ruse to lure the Doctor, and that Morgaine has summoned the Destroyer of Worlds. Morgaine appears before Ace and Shou Yuing, and tries to entice them to hand over Excalibur. When they refuse, she unleashes the Destroyer.
The Doctor and the Brigadier capture Mordred, and set off for the hotel. Meanwhile, Morgaine is occupied with Ace's circle. On return, the Doctor finds the hotel in ruins yet Ace and Shou Yuing safe. He is pleased to hear that Ace gave up Excalibur to Morgaine, if doing so protected her. In the debris, the Doctor finds a portal to Morgaine's castle; he, the Brigadier, and Ace enter. On arrival, the Brigadier shoots the Destroyer, to no effect. The Destroyer's return volley sends the Brigadier flying through the window. Ace bursts through the portal, ramming into Morgaine, knocking Excalibur from her grasp.
Morgaine releases the Destroyer's bonds. In the confusion, she scoops up Excalibur and enters the gateway along with Mordred.
Outside, the Doctor readies the Brigadier's revolver and silver bullets — yet the Brigadier knocks him out, retrieves his gun, and faces the Destroyer alone. After some dialog, the Brigadier empties the revolver into the Destroyer's chest. The Doctor wakes to find the castle engulfed in flames. He spots the Brigadier's prostrate form, and begins to mourn his fallen friend — at which point the Brigadier stirs and rises, scuffed but unharmed.
Back at the convoy, Morgaine and Mordred attempt to detonate the nuclear missile. The Doctor confronts Morgaine, insisting that there is no honor to nuclear warfare. She agrees, then asks to fight Arthur in single combat. He tells her of Arthur's death, to her sadness. The Doctor prevents Mordred from killing Ancelyn, and asks Bambera to imprison Mordred and Morgaine.
Back at the Brigadier's house, Doris thumbs her nose at her husband by going for a drive in Bessie with Ace, Shou Yuing, and Bambera, leaving him and the other men to do the gardening and prepare supper.
Doctor Sylvester McCoy (Seventh Doctor)
Companion Sophie Aldred (Ace)
Nicholas Courtney — Brigadier Lethbridge-Stewart
Angela Douglas — Doris
Angela Bruce — Brigadier Winifred Bambera
Robert Jezek — Sergeant Zbrigniev
Dorota Rae — Flight Lieutenant Lavel
Paul Tomany — Major Husak
Noel Collins — Pat Rawlinson
June Bland — Elizabeth Rawlinson
James Ellis — Peter Warmsly
Ling Tai — Shou Yuing
Jean Marsh — Morgaine
Christopher Bowen — Mordred
Marcus Gilbert — Ancelyn
Marek Anton — The Destroyer
Stefan Schwartz — Knight Commander
Writer Ben Aaronovitch
Director Michael Kerrigan
Script editor Andrew Cartmel
Producer John Nathan-Turner
Executive producer(s) None
Production code 7N
Series Season 26
Length 4 episodes, 25 minutes each
Originally broadcast September 6–September 27, 1989