Battlefield is the first serial of the 26th season of the British science fiction television series Doctor Who, which was first broadcast in four weekly parts on BBC1 from 6 to 27 September 1989. It was the last to feature Nicholas Courtney as Brigadier Lethbridge-Stewart in the show.
In the serial, Morgaine (Jean Marsh), a sorceress from another dimension, summons the planet-devouring Destroyer (Marek Anton) in England, where she also seeks to take the sword Excalibur for herself. The plot is loosely based on Arthurian legend.
Plot
In response to a distress signal, the Seventh Doctor and Ace materialise the TARDIS near Lake Vortigern in England, where a UNIT convoy led by Brigadier Winifred Bambera is transporting a nuclear missile. At the Gore Crow hotel, they meet a young woman called Shou Yuing and a knight, Ancelyn, who addresses the Doctor as "Merlin". The Doctor shows interest in a scabbard which hangs over the mantelpiece in the hotel. The party is surrounded by an ominous group of knights led by Mordred who leaves after recognising The Doctor as "Merlin". When Mordred begins an arcane ritual, the scabbard flies across the room, stirred by the magic. A sorceress, Morgaine, then arrives on the scene through a rift in space and time.
The next day, archaeologist Peter Warmsly shows the Doctor where he uncovered the scabbard. Under the lake, The Doctor and Ace find a ruined spaceship containing the body of King Arthur, lying next to a sword. When Ace removes the sword from its plinth, she activates a defence mechanism. The Doctor ejects Ace from the spaceship, sending her shooting up through the water, before the Doctor is rescued by Brigadier Lethbridge-Stewart.
Mordred and Morgaine go to the hotel to retrieve Excalibur. Meanwhile, the UNIT troops are staging an evacuation. Just as Mordred and Ancelyn are about to fight, the Doctor intervenes. Mordred reveals that the battle was a ruse to lure the Doctor, and that Morgaine has summoned the Destroyer of Worlds. The fighting between Morgaine's armies and the UNIT convoy continues, with UNIT victorious.
In an attempt to coerce Ace into surrendering Excalibur, Morgaine eases her control over the Destroyer, who begins to destroy the Hotel. The Doctor finds the hotel in ruins, but Ace and Shou Yuing safe. In the debris, the Doctor finds a portal to Morgaine's castle. On arrival, the Brigadier shoots the Destroyer, to no effect. Morgaine releases the Destroyer's silver bonds; afterwards she and Mordred escape without Excalibur back through the portal.
Lethbridge-Stewart, recalling a comment made by the Doctor, heads back into the castle armed with silver bullets. He confronts the Destroyer, and empties his revolver into the monster's chest, destroying it and Morgaine's castle. Despite the ominous destruction, Lethbridge-Stewart survives. They, with Ancelyn, return to the ship to use Excalibur to revive King Arthur, but discover a letter from "Merlin" that the King died in a final battle.
Back at the convoy, Morgaine and Mordred seize control of the nuclear missile in an attempt to set it off. The Doctor confronts Morgaine, tells her the dishonourable horror of the missile, and of Arthur's death. She relents, defeated, and she and Mordred are then imprisoned.
The group then return to Lethbridge-Stewart's home after the battle is over, but find that it is just beginning when Doris, Lethbridge-Stewart's wife takes Ace, Shou Yuing and Bambara out on the road.