The Doctor and Morgaine

In response to a distress signal the TARDIS arrives on Earth near Lake Vortigern in England in the near future. The Doctor and Ace, his travelling companion, discover by the lake an archaeological dig, headed by Peter Walmsley of the Carbury Trust. They also discover a UNIT convoy transporting a nuclear missile which has stopped near the dig due to technical problems. In charge of the UNIT convoy is Brigadier Winifred Bambera who is told, by Sergeant Zbrigniev, about The Doctor and his ability to change his appearance and personality. But she does not believe him.

Brigadier Bambera gives the two time travellers a lift to the Gore Crow Hotel, where Ace befriends a Chinese girl called Shou Yuing. In the hotel they discover a scabbard, which has been excavated from the battlefield, hanging above the mantelpiece in the hotel lounge. As The Doctor inspects the scabbard he discovers that it is hot to his touch. Elizabeth Rowlinson, the hotelier’s blind wife, also seems to sense something strange about it – as though it is waiting for something. Leaving The Doctor inside the hotel Ace and Shou Yuing retire to the garden to talk about explosives.

Nearby several armoured knights have crashed to Earth from space, and one of them finds the TARDIS. The knights though start to fight amongst themselves using both swords and futuristic guns. As they continue to clash a grenade is thrown and one of the knights is blown up in the air and through the roof of a brewery behind the hotel, startling Ace and Shou Yuing. The Doctor and the girls investigate and find inside the brewery a knight who introduces himself as Ancelyn. The knight also greets The Doctor as ‘Merlin’. Brigadier Bambera then arrives, her car having been damaged in the fighting. Finally Mordred enters, flanked by knights. The Doctor prevents Mordred and Ancelyn from fighting and, taking advantage of the fact that both believe him to be Merlin, he orders Mordred to leave. The knight complies, but tells The Doctor that his mother has waited twelve centuries to face him. Mordered then enters an abandoned church and summons his mother - a powerful sorceress called Morgaine. She appears and speaks to The Doctor through a mental link, warning him not to stand against her.

Lethbridge-Stewart and Doris

Meanwhile Alistair Gordon Lethbridge-Stewart (formerly known as The Brigadier) is enjoying his retirement from UNIT, with his wife Doris, when he receives a phone call and is directed, by UNIT commanders in Geneva, to go to London. On his way there he orders that an exclusion zone to be placed around Carbury. The next morning, The Brigadier travels from London to Carbury in a helicopter piloted by Flight Lieutenant Lavel. But they are unaware of Morgaine who is in a churchyard with her knights and, on seeing The Brigadier’s helicopter, she makes it crash. The Brigadier though is unharmed and he goes to fetch help for Flight Lieutenant Lavel, who is injured. He encounters Morgaine but they part with a mutual understanding. He then hurries to the hotel and commandeers Shou Yuing’s car from her.

At the archaeological site Peter Walmsley shows The Doctor and Ace where he uncovered the scabbard. They find a rune, written in The Doctor’s own handwriting, which he translates to read: ‘Dig hole here’. Using a canister of Nitro-9 explosives, Ace creates a large hole in the ground that reveals a tunnel leading downwards. They enter and eventually find themselves in a spacecraft under the lake. There they discover the slumped, armour-clad figure of King Arthur, lying next to a sword, which was apparently the source of the distress signal, and also lured Mordred and Morgaine. However, before The Doctor can stop her Ace pulls the sword from its mounting. In doing so she inadvertently unleashes a hostile, glowing, snake-like entity. This defence mechanism knocks The Doctor unconscious and Ace finds herself trapped in an airlock as it rapidly fills with water. The Doctor recovers just in time to eject Ace from the space ship, sending her shooting up through the waters to the surface of the lake. There Peter Walmsley and Ancelyn, who are standing at the shore discussing the Lady of the Lake, witness Ace emerge from the water still grasping the sword. Ancelyn immediately identifies it as Excalibur. The Brigadier then arrives, along with Shou Yuing, and he heads down the tunnel where he destroys the creature so rescuing The Doctor.

Morgaine

Morgaine, aware of the Excalibur’s discovery sends her Knight Commander to retrieve the sword from Ace. Meanwhile Mordred is drinking at the hotel when Flight Lieutenant Lavel enters and holds him at gun-point. Morgaine arrives and forces Flight Lieutenant Lavel to lower her weapon and kneel at her feet - before killing her by draining the knowledge from her mind. When the hotelier Pat Rowlinson complains, she disintegrates the body and pays for the drinks by restoring Elizabeth Rowlinson’s sight.

The Doctor, Ace, Shou Yuing, Brigadier Bambera, The Brigadier, Ancelyn and Peter Walmsley all return to the hotel, trying to avoid Morgaine’s knights along the way. There, UNIT troops take steps to evacuate all the civilians. Major Husack though encounters resistance from Pat Rowlinson and Peter Walmsley, but The Doctor convinces the two men that they wish to leave and they do so with no further problem. The Brigadier has ordered up numerous weapons - and adds to the inventory, at The Doctor’s suggestion, silver bullets. He has also had Bessie, The Doctor’s yellow car, brought to the hotel. The Doctor then heads off for the missile convoy in order to prevent a battle there. He leaves Ace with some chalk, instructing her to draw a circle and stay inside it, with Shou Yuing and the sword, as it will protect them against Morgaine's sorcery. But soon afterwards a storm breaks outside the hotel and Morgaine, after having watched this exchange through her crystal ball, transports herself to the hotel, where she discovers Ace and Shou Yuing cowering in the circle. Unable to cross the chalk Morgaine attempts to obtain the sword by playing mental tricks on the young women and then, when this fails, she threatens them with the prospect of releasing a blue demon, whom she has bound with silver chains, that is known as the Destroyer of Worlds.

The Doctor manages to bring a temporary halt to the fighting at the convoy. Mordred is captured and reveals that The Destroyer has been summoned. The Doctor tries to get Morgaine to stop what she is doing by threatening Mordred’s life but, to Mordred’s horror, she is undeterred. Morgaine though is still unable to touch the sword so long as it is in the chalk circle. The Doctor hurries back to the hotel, but there is a massive explosion just before he arrives. Ace and Shou Yuing are unharmed, but Ace admits that she gave Morgaine the sword. But rather than being angry The Doctor is pleased to hear that Ace gave Excalibur to Morgaine, as doing so protected her. In the debris, The Doctor finds a curtain of light which is a portal to Morgaine's castle. The Doctor and The Brigadier pass through it and find Morgaine and The Destroyer waiting for them. The Destroyer reveals that it enabled The Doctor to come here in the hope that Morgaine would be forced to free it. The Brigadier attempts to shoot The Destroyer but is knocked through a window by the creature.

The Destroyer

Ace has meanwhile realised that The Doctor may need the silver bullets and so bursts through the portal, ramming into Morgaine, and knocking Excalibur from her grasp. This gives The Doctor the chance to grab the sword but Morgaine frees The Destroyer and regains the sword. She and Mordred, who has arrived from the battlefield, then fade away, while The Doctor and Ace beat a hasty retreat with The Brigadier, who it is discovered is unharmed.

Outside, Ace tells The Doctor that Morgaine bound the Destroyer with silver chains and so he loads The Brigadier's revolver with the silver bullets that Ace brought with her. Although The Doctor wants to face The Destroyer himself, The Brigadier, realising that he is too valuable, knocks The Doctor out and on retrieving his gun he marches back in to the castle to face The Destroyer in The Doctor's place. After emptying the revolver into the monster's chest The Destroyer explodes. The Doctor then wakes to find the castle engulfed in flames. He then spots The Brigadier’s prostrate form, and begins to mourn his fallen friend. But it turns out however, that The Brigadier is not dead after all, only unconscious.

Morgaine and Mordred meanwhile, having returned to the UNIT convoy, plan to detonate the nuclear missile. The Doctor and Ace revisit the spacecraft under the lake where Ace replaces the sword in its mounting. The Doctor discovers that the suit of armour, in which Arthur is supposedly held in suspended animation, is empty but for a note - left for him by his own future self - explaining that Arthur died in the final battle and warning that Morgaine has taken control of the nuclear missile. As Ancelyn and Mordred fight, The Doctor goes to stop Morgaine. He convinces her that to use the nuclear missile is dishonourable. She realises that he is right and she hits the abort button. Morgaine then asks to fight King Arthur in single combat. The Doctor is forced to tell her of his discovery under the lake and that King Arthur died many years ago. This news distresses Morgaine who, along with Mordred, are imprisoned by Brigadier Bambera.

With Morgaine and Mordred defeated, and back at The Brigadier’s house, Ace, Shou Yuing, Brigadier Bambera (who has developed a relationship with Ancelyn) and Doris set off for a drive in Bessie. This leaves The Brigadier and Ancelyn to do the gardening and The Doctor to cook supper.