Little Hodcombe, 1984 : schoolteacher Jane Hampden searches for her farmer friend Ben Wolsey, but is attacked by soldiers in English Civil War uniforms. She takes shelter in the barn, only to be confronted by a Roundhead officer -- who turns out to be Wolsey. The village is in the midst of a war game organized by the Squire, Sir George Hutchinson, but Jane believes that the high spirits are getting out of hand and demands that the games be stopped. Hutchinson, who is constantly toying with a strange metal sphere, refuses; there is only one battle left to fight. What's more, he has isolated the village -- until the games are over nobody will be allowed to enter or leave...

The Doctor takes Tegan to Little Hodcombe to visit her grandfather, but just as they are about to materialize the ship encounters interference from some sort of energy field. They materialize in the crypt of the local abandoned church, and when the scanner shows a figure apparently being trapped by falling debris, the Doctor rushes out to offer help. The figure flees upstairs, and the Doctor follows -- while Turlough and Tegan try to figure out why someone in the year 1984 would be wearing the rags of a 17th-century beggar. The beggar has gone by the time they reach the church, and the Doctor notices a woodcut of a man being chased by the Devil near a large crack in the wall. As he and his friends leave, smoke starts to pour from the crack...

Outside, they are confronted by Roundhead soldiers led by the sadistic "Sergeant" Willow, who takes them to Wolsey's home for interrogation. In a room which Wolsey has designed down to the last detail as a perfect representation of the 17th century, the Doctor and his companions try to find out what's going on -- and when Tegan identifies herself as Andrew Verney's granddaughter, there is a sudden silence. Verney disappeared some days ago and nothing's been done to find him. Tegan, frustrated, rushes out of the room with Turlough in pursuit -- and Willow holds the Doctor at gunpoint to prevent him from leaving.

Tegan hides near the barn, but the beggar from the church -- a man with a terribly burnt face -- grabs her purse and vanishes into the barn with it. Tegan follows, but the door slams shut behind her, and an apparition appears in the loft -- sparkling lights surrounding a disembodied torso which is then replaced by the spectral head of an emaciated old man. Turlough finds her just as the apparition vanishes, and they retreat back to the TARDIS as quickly as possible.

Hutchinson arrives to question the Doctor, and explains that the village is recreating the battle of 13 July 1643 which utterly destroyed Little Hodcombe at the time. The Doctor is beginning to understand Jane's objections, and comes to realize how obsessed with the games Hutchinson is. Hutchinson warns the Doctor to stay away from the church, but the Doctor surprises him and Willow and escapes. Willow tells Hutchinson that Tegan is Verney's grand-daughter, and Hutchinson orders him to find her -- he has something special in mind for her...

The Doctor, searching for Tegan and Turlough, sees the beggar run by holding Tegan's purse -- but the beggar puts on an impossible turn of speed and outraces the Doctor, back to the church. In the church, the Doctor hears the sounds of battle and smells gunpowder. The noise nearly overpowers him -- until a young boy smashes his way out of hiding in the church wall. The boy, Will Chandler, seems to have come from the year 1643, having hidden from the fighting. Tegan and Turlough arrive, and the Doctor tells them that some alien power source has linked the year 1643 with 1984 and is causing psychic manifestations. And the crack in the church wall has grown larger. He sends Tegan and Turlough back to the TARDIS to safety while he and Will investigate back at the village. Tegan and Turlough arrive at the TARDIS only to find that the sparkling lights have invaded the console room as well, and they head out of the church to find the Doctor and warn him.

Unfortunately, the Doctor never made it to the village; while showing Will the tombstones in the churchyard, and the dates which prove that Will has travelled centuries into his future, soldiers arrived and he and Will were forced to take shelter in the vestry. There, Will flies into a panic when he sees a carving of the Devil on a flagstone, and the Doctor questions him and learns that Will saw a figure he calls the "Malus" in 1643 -- a demon which made the fighting worse. The Doctor leans on the flagstone and it opens up to reveal a secret tunnel.

Willow and Wolsey are unable to find Tegan, and Hutchinson heads out to help organize the search. Jane plans to contact the police and put a stop to this; things have gone too far, and she suspects that Willow and Hutchinson intend to resurrect the old traditions about the village May Queen. Willow reminds her that she can still become May Queen if Tegan isn't found, and locks her in Wolsey's study. Jane eventually finds a secret panel hidden behind a tapestry, and flees just as Sir George enters the room. In the tunnel behind the panel, she encounters the Doctor and Will, and they hide beneath the stairs as Hutchinson and his soldiers pursue Jane. Hutchinson, unable to find her, returns to the village to complete his preparations for the final battle. Jane realizes that this must be the passage which Andrew Verney, the local historian, mentioned discovering before he vanished. The Doctor has more esoteric concerns; a sample of the metal which Hutchinson is constantly toying with has fallen to the floor, and the Doctor recognizes it as tinclavic -- which is only found on the planet Raaga.

Turlough and Tegan search for the Doctor in the village, but split up to avoid a patrol of soldiers. Tegan runs right into Wolsey's hands, and he turns her over to Willow, who takes her to another building to change into period clothing. He threatens to do it for her if she refuses, and she reluctantly changes into the costume of the village's Queen of the May...

The Doctor emerges from the smoke, shaken, as the rest of the wall falls away to reveal the face of the Malus -- the same face representing the Devil on the woodcuts in the church. The beggar reappears, surrounded by sparkling lights, a psychic projection from the Malus -- as is the sound of gunfire and battle which now fills the church. Will flees in terror as the beggar transforms into the image of a ghostly Cavalier soldier. An ethereal wind fills the church as the ghost approaches the Doctor and Jane, who flee back to the underground passage.

Turlough hides near the village square and sees soldiers preparing a Maypole for the Queen. He is then captured and locked up in a nearby barn, where he meets a fellow prisoner -- Andrew Verney. Verney found the dormant Malus in the church, but made the mistake of telling Sir George Hutchinson, whose deranged mind brought it back to life. He and Turlough try to combine their strength to batter down the locked door.

The Doctor and Jane head back to Wolsey's house to try to talk sense into Hutchinson. He must be the medium through which the psychic energy generated by the war games is being focussed to feed the Malus; but if he is doing this deliberately he has underestimated the Malus' capacity for evil and destruction. The Malus must have lain dormant for quite some time until the fighting of 1643 generated enough psychic energy to temporarily awaken it. Now Sir George is trying to recreate the same events and bring the Malus to life -- and this means that the final battle in the war games, the one which destroyed the village in 1643, will be for real.

Wolsey guards Tegan to prevent her escape, and when Hutchinson arrives he informs her that this is no game; she is participating in an event which will change the course of history. The Doctor arrives and urges Hutchinson to stop the games, but he refuses and departs, ordering Wolsey to kill the Doctor. Wolsey no longer understands Hutchinson's obsession, until a psychic projection materializes in his room -- an extension of the Malus, gathering psychic energy and transferring it to the parent in the church. The Doctor leaves to find Turlough and Will, while Wolsey advises Tegan to remain in her May Queen costume for the moment; if she doesn't turn up on time Hutchinson will turn out the entire village to search for her. Meanwhile, in the church, the Malus is growing ever stronger, pushing its way out of the wall...

The Doctor finds Will hiding near the village square, terrified that everything is happening just as it did before -- when they burned the May Queen alive at the stake. The Doctor confronts the soldiers but is held captive and must watch as Wolsey's cart approaches, with the May Queen in the back. But when it actually arrives, Hutchinson and Willow discover that the "May Queen" is in fact a straw dummy; Wolsey has allowed Tegan and Jane to escape. Hutchinson orders Willow to kill the "traitor", and in the confusion which follows, Will grabs a flaming torch and attacks the guards holding the Doctor. Hutchinson's horse panics and throws him, and the Doctor, Will and Wolsey escape on the May Queen float. The Malus, thrown by the disruption, grows dormant.

The Doctor, Will and Wolsey reach the church and are reunited with Tegan and Jane. They run past the Malus into the crypt, where the Doctor finds another Malus extension in the TARDIS, growing out of the lights Tegan and Turlough saw earlier. Willow arrives with another soldier, and Tegan closes the TARDIS doors while the Doctor tries to lock onto the Malus' psychic frequency. He is unable to control the Malus as he had hoped, but does manage to cut it off from the source of its power, the psychic energy in the village. The extension in his TARDIS, cut off from its source, begins to disintegrate into an organic slime. The Malus responds by summoning Sir George, who is overwhelmed by the psychic energy of the Malus and rushes off to the church with an agonised cry...

Turlough and Verney manage to smash their way out of the barn and head for the church, where Verney is appalled by the size of the Malus -- it's far too late to destroy it in this form. Turlough hears sounds from the crypt as Willow and the soldier try to break into the TARDIS, and he and Verney enter the crypt and overpower them. The Doctor and the others then emerge from the TARDIS and head upstairs to plan their next move, but the Malus generates projections of ghostly Roundhead soldiers, cutting off their escape. The ghosts approach slowly, swords drawn; the Malus is deliberately extending the suspense in order to feed off its victims' fear. At the last moment the soldier from the crypt recovers and staggers upstairs, directly into the swords of the ghosts.

Before the others can escape, Hutchinson arrives and holds them at gunpoint. Wolsey and the Doctor try to talk sense into him, but he's under the total domination of the Malus. The Doctor tries to point out that Hutchinson has driven his village into turmoil and is pointing a gun at a man who was his friend; the Malus is pure evil and will destroy Hutchinson as soon as it has no further use for him. As Hutchinson wavers, confused, the Malus summons Willow out of the crypt to attack, and in the confusion, Will rushes forward and pushes Hutchinson into the gaping maw of the Malus. The Malus, realizing that it has lost without its medium, prepares to fulfil its programming and destroy everything within reach, and the others flee back to the TARDIS and escape as the Malus explodes, destroying the church.