Battlefield was the first story in season twenty-six of Doctor Who, which would ultimately be the last season of the original 1963-89 series.
Nicholas Courtney made his first appearance as Brigadier Lethbridge-Stewart since 1983’s The Five Doctors, and Bessie appeared for the first time since that serial, as well. This was also the first story to feature UNIT since The Seeds of Doom (bar a brief appearance in The Five Doctors) and also the last of the classic series — UNIT would be featured once again in Aliens of London in 2005. Jean Marsh, who played Sara Kingdom in the 1960s, returned in a different role. The Seventh Doctor’s costume was also modified as of this story, shifting toward darker accents of clothing, along with his darkening personality.
Interestingly, Battlefield introduced the idea that the Doctor was the man behind the myth of the wizard Merlin at some point in his life. Since this ended up being one of the final stories of the classic series, the full details behind this mysterious plot line were never worked out on television and instead interpreted differently across the Doctor Who expanded universe as multiple writers tried their hand at explaining these untold events.
Brigadier Lethbridge-Stewart, now retired from teaching as well as UNIT, is enjoying retirement with his wife Doris. UNIT may still need the gallant old soldier, though; new Brigadier Winifred Bambera and her soldiers, in a nuclear missile convoy near rural Carbury, are faced with inexplicable equipment failure and peculiar sightings of armor-clad knights. In the excitement, the UNIT convoy ignores a pair of hitchhikers nearby: the Seventh Doctor and Ace, lured to Carbury by a distress signal so powerful that it even crosses universes.
The Doctor and Ace eventually hitch a ride with archaeologist Peter Warmsly, who is excavating an old battlefield near Carbury — though even he is surprised to find his dig site taken over by tight-lipped UNIT soldiers. While he attempts to get an explanation, the Doctor unearths old UNIT identification (his own and Liz Shaw’s) to get himself and Ace into the UNIT camp. Bambera, nonplussed, confiscates the passes and ejects the two travelers from the convoy. But one UNIT soldier, who had previously served under Lethbridge-Stewart, recalls UNIT’s former scientific advisor: his eccentricity, his ability to change appearance, and his tendency to attract trouble.
Intrigued, Bambera gives the Doctor and Ace a lift to the nearby Gore Crow Hotel… and asks UNIT headquarters to recall Lethbridge-Stewart to duty.
At the hotel, Ace meets a kindred spirit in Shou Yuing, while the Doctor talks to innkeeper Elizabeth Rowlinson. She is blind but psychic, and knows that the ancient scabbard over the inn’s fireplace — an artifact found by Warmsly — is an object of tremendous and mysterious power.
Ace and Shou are chatting outside the hotel when they and the Doctor witness an oddly human-shaped missile crash into innkeeper Pat Rowlinson’s microbrewery. It is revealed to be a knight in full armor — not an android, as Ace initially thinks — and, when his helmet is removed, he warmly greets the Doctor as “Merlin”.
The Seventh Doctor, Ace, and Shou chat with the knight, Ancelyn, who refers to the distress signal intercepted by the TARDIS as “Excalibur’s call” and claims that it heralds the start of a final war and King Arthur’s restoration to the throne. The Doctor surmises that the coming battle isn’t properly part of this dimension at all, and belongs to a parallel universe — but before he can test the theory, an exasperated Brigadier Bambera storms into the microbrewery and declares everyone under arrest. She is interrupted by the unexpected arrival of more extra-dimensional knights, including Ancelyn’s sworn enemy Mordred, who also recognizes the Doctor as Merlin. After Mordred retreats, unwilling to face the Doctor, Ancelyn and Bambera vie for dominance and the rest of the party retreats to the Gore Crow’s lounge.
Mordred, with the help of a sword he calls “brother to Excalibur”, begins a strange ritual intended to link his home dimension to present-day Carbury and allow his mother Morgaine to cross over. As he roars in triumph at the ritual’s success, the scabbard on the wall of the Gore Crow’s lounge breaks free and sails across the room — towards the lake — before implanting itself in a wall, distressingly close to Warmsly. The Doctor realizes what is happening, but cannot stop it… Morgaine crosses over, and the hotel is
plunged into darkness. Morgaine and Mordred discuss their plans for battle; the sorceress, seeing a helicopter approach, demonstrates her power by bringing it down. The helicopter, which is bringing Lethbridge-Stewart to Carbury, manages to land, and Lethbridge-Stewart sets off in search of the Doctor.
Warmsly shows the Doctor and Ace the Carbury dig, and points out a mysterious inscription. The Doctor recognizes his own handwriting instructing him to dig a hole, which he asks Ace to do with some well-placed explosives. At the bottom is an 8th century tunnel… made of cement. The Doctor and Ace investigate, having left Warmsly to guard them.
At the end of the tunnel is a strange room. Ace and the Doctor look in vain for some way out, until the Doctor simply commands the door to open, and it obeys. As they proceed to the next rooms, the Doctor explains that Merlin must have built the rooms — spaceship, it now appears — that they are in… and he is
Merlin, or will be. In the topmost level of the spaceship is King Arthur, apparently in suspended animation, and Excalibur. Ace accidentally draws Excalibur, which triggers an automated defense system that traps her in an airtight chamber rapidly filling with water, and renders the Doctor helpless as she begins to drown…
As the two wander near Lake Vortigern, Warmsly quotes Malory’s “Morte d’Arthur” to Ancelyn… when, to his astonishment, a hand appears from the lake brandishing Excalibur. It proves to be Ace, whom the Doctor managed to free from the spaceship below the lake. She emerges wet but none the worse for the wear, and is joined by Warmsly, Ancelyn, Bambera, and, arriving with Shou in her car, which he has commandeered, Lethbridge-Stewart. The Doctor continues to fight the spaceship’s defence system, but it is only stopped when Lethbridge-Stewart, who has entered through the tunnel, intervenes. Reunited, the friends leave by the tunnel and rejoin the others at the dig.
Meanwhile, Morgaine has sent Mordred and his knights after the Doctor and the others, with orders to take Excalibur and kill any who resist. The Rowlinsons are terrified by Morgaine’s cavalier killing of Flight Lieutenant Lavel, but grateful — if bewildered — when she restores Elizabeth’s sight.
The party splits up for the ride back to the hotel: Bambera and Ancelyn in Shou’s car; the Doctor, Ace, Shou, and Warmsly in his. Mordred’s knights set up an ambush in the woods between the hotel and the dig, but ultimately attack the wrong car; the others make it back to the hotel safely, with Excalibur. There they find that all locals are being evacuated — Shou and Ace slip away unnoticed, while Warmsly and the Rowlinsons protest but ultimately agree to evacuate.
A UNIT patrol sent to find Bambera and Ancelyn reports that the two soldiers escaped the knights and are at large in the woods. The news, though welcome, does not entirely cheer the Doctor, who points out that Mordred and his knights are also at large… and that nothing stands between them and the now-unguarded missile convoy by the lake. They must return to the convoy immediately. Fortunately for the Doctor, Lethbridge-Stewart thought to bring along transportation: Bessie, the Doctor’s car from his UNIT days. He and the Doctor depart, leaving Ace and Shou behind with Excalibur and instructions on avoiding Morgaine’s sorcery.
Morgaine leaps at her chance to snatch Excalibur from the hands of its young guardians, summoning the Destroyer to help her, but Ace remembers the Doctor’s warning and draws a protective circle of chalk. Much to her surprise, it seems to work… until Morgaine tries a more subtle psychological attack, and sets Ace and Shou fighting, trying to draw one or both out of the circle.
At the dig, the ancient battlefield has become a modern one, as UNIT troops face off against Mordred and his knights, and Mordred himself is confronted by Ancelyn — but as they prepare to fight, the Doctor intervenes and declares that there will be no more bloodshed. But Mordred smugly announces that the battle was only a diversion for his mother Morgaine’s attempt at Excalibur… and that Ace and Shou stand no chance against Morgaine and the Destroyer.
Mordred offers the Doctor a trade — the girls’ lives for Excalibur — but the Doctor, outraged, threatens to decapitate him unless Morgaine surrenders herself. But Mordred calls the Doctor’s bluff; he knows, he says, of Merlin’s distaste for taking life. He is proved right, but Lethbridge-Stewart appears out of the woods, gun ready, and declares that he has no such compunctions. The two old friends force Mordred into their car and head back to the Gore Crow Hotel.
They arrive just as the Destroyer, under Morgaine’s command, has nearly brought the hotel down on Ace and Shou. The Doctor finds his friends under a pile of rubble, relieved that they were able to stay alive, even at the cost of surrendering Excalibur to Morgaine.
She, meanwhile, has returned, with Excalibur and Mordred, to her castle in her reality. The Doctor and the Brigadier follows her across the void; he himself is followed by Ace, who brings along Excalibur’s sheath and the Brigadier’s silver bullets.
Morgaine frees the Destroyer, to the Doctor’s dismay. While she and Mordred teleport away, out of the Destroyer’s immediate path, the Doctor and his two friends are left to flee on foot. The Doctor deduces that, since the Destroyer was bound in silver chains, it will be vulnerable to the Brigadier’s silver bullets. Lethbridge-Stewart distracts the Doctor, knocks him out and takes the gun — loaded with the silver bullets — to spare the Doctor the duty of killing the Destroyer. When the Doctor revives, he and Ace find that Lethbridge-Stewart has done so, nearly paying for the opportunity with his life.
Even without the Destroyer, Morgain and Mordred still pose a threat, as they prove when they cross back into our world and take Bambera hostage. They force her to divulge the launch codes for the nuclear missile, and prepare it for launch.
Ancelyn, the Doctor, Lethbridge-Stewart, and Ace return Excalibur to the spaceship under the lake. There they find that the sword activates the spaceship — but does not resurrect Arthur. According to a note left by the Doctor for himself, Arthur was killed in the Battle of Camlann, not placed in suspended animation. The note also warns them that Morgaine has control of the nuclear missile, and the four friends rush back to the surface.
While Ace and the Brigadier destroy the spaceship, the Doctor tries to stop the missile launch. Morgaine is obstinate… but the Doctor explains the full horror of nuclear warfare, and she relents with seconds to spare. She demands of the Doctor that King Arthur face her for a final confrontation, but the Doctor reveals that Arthur died in his last battle. Grief-stricken, she does not notice the Doctor leave.
The Doctor intervenes in a sword fight between Ancelyn and Mordred, and renders Mordred unconscious. Ancelyn is freed, and Mordred and Morgaine surrendered to UNIT.
Back at the Lethbridge-Stewart house, the Brigadier and Ancelyn are left to work in the garden while the Doctor cooks supper. In the meantime, Doris, Shou, Ace, and Bambera have commandeered Bessie for a girls’ trip out on the town.