Somewhere in London a Krashok warship has landed in a ruined warehouse and six nightmare creatures have emerged. The Doctor, hidden in the shadows, watches in horror as they descend the ramp. The creatures were once human to look at but have become an ugly jigsaw of human, robot ad alien, each different to the others. The Krashok travel the universe in search of fearsome predators and fighters. They kill them and take their weapons which they add to their armoury. The Doctor gazes at the six aliens and recognises parts from at least eleven other species: Renevian tiger claws, Slitheen arms, the legs of a Gappa. He knows that their insides will be similarly varied as they acquire lungs, hearts and stomachs that allow them to live and fight in almost any conditions.

The warriors have helmets bolted to their heads containing laser sights and carry an array of weaponry from all over the galaxy. These include Dalek and Cybermen guns. The Doctor knows that the Krashoks business involves starting wars and then selling weapons to both sides. He knows that the Eternity Crystal will allow them to keep bringing back the dead from the battlefields and that the next war will be interminable.

Two commanders emerge from the ship and descend towards the Darksmiths waiting for them. One of the commanders greets Drakon by name while the other simply demands the Crystal. One of the commanders, Skraar, reminds Drakon that the millennia delay in delivering the Crystal has cost them a lot of money. Drakon waves at Gisella who steps forward and opens a box, revealing the Crystal. Drakon reminds Skraar that payment is due and the Krashok smiles his agreement.

The Doctor knows he has to prevent the transaction from being completed. He pulls out his sonic screwdriver and aims at the twisted and rusty beams in the ruined roof above. He glances back at the meeting and sees Drakon being presented with a large chest. The Doctor sends a sonic beam towards the roof, causing the beams to tremble. Drakon throws open the lid of the chest with a triumphant smile which soon fades as he realises that the chest is empty. Commander Grelt informs him that this is what he deserves for being so late with the delivery. When Drakon demands payment the six Krashok troopers point their weapons and kill Drakon and the other Darksmiths. There is only Gisella alive, holding the crystal. Grelt takes it from her and then points his gun at her.

It is at this moment that the beams finally give way and the remains of the roof begin to fall on top of the Krashoks. As the aliens scatter the Doctor runs to Gisella. She is unsurprised to see him, saying that she was expecting him. The Doctor demands to know why he shouldn’t leave her there in the path of falling masonry after she handed over the Crystal but she tells him she didn’t give the Krashoks the real Crystal; it was the fake that her father made.

Heartened by this, the Doctor grabs her hand and they scamper through the rain of debris to the TARDIs. Their path is blocked by Commander Skraar but before he can kill them a huge block of masonry crushes him. As they reach the TARDIs they hear the Krashok ship’s engines throb into life.

Inside the ship Commander Grelt is in a foul mood when he is told that Skraar is dead. He orders the pilot to lift off but is told that there is too much debris weighing the ship down. The primary engines have been damaged, too. He orders the pilot to use the emergency temporal shift they took from the Daleks.

The Doctor recognises the blinding light as the ship vanishes. Bundling Gisella into the TARDIS, he tells her that the Krashoks won’t have travelled far in space, only a few miles, but they will have gone several hundred years backwards or forwards. He uses the TARDIS scanners to track them to Medieval London. The problem is that the fake crystal, when fitted to the Krashok machine, could cause an explosion that will destroy the entire planet.

Oswald the bard is crossing the snowy fields several miles outside London. As night begins to fall he reflects on the argument over a girl that saw him leave the nearby village in a hurry. His only options are to make for London or Sir Howard Blackheart’s castle. Deciding he does not want to arrive in London for the first time in such a bedraggled condition he elects to go to the castle. This is despite the terrible stories he has just heard of lights in the sky and a plague that killed instantly the men in the fields around the castle. Not wanting to be out in the dark he hurries on his way but is stopped short by the sudden trumpeting and blue lights that signify the arrival of the TARDIS in his path. The sight of the Doctor and Gisella stepping out into the snow causes Oswald to faint.

Grelt makes his way through a tunnel. He reflects on the value of Sergeant Jorak’s advice, stopping him from killing all the humans in the field after they arrived. Now the humans are being used as labour to help in the repairs to his ship and in seventy two hours the Krashoks should be on their way again. He arrives at the end of the tunnel in a cave where an animal is being roasted over a fire. An old woman seems to be waiting for him. Alone of all the humans he has met she does not fear him. Instead, she recognises that he has come from the future. Moreover, she tells him that Skraar’s killer has also just arrived from the future. Sensing his chance for revenge, Grelt asks where the Doctor is now.

Oswald wakes up in the TARDIS. Despite his awe he is more excited than afraid. He introduces himself as a travelling minstrel and says that he only fainted when he thought that the Doctor and Gisella were the demons he had heard about earlier that day. The Doctor demands to know what else he can tell them and he speaks of the demons that arrived in a sudden storm and the sudden deaths of the men in the field. The Doctor asks where the demons are and demands that Oswald leads them to the castle.

Grelt gleefully tells Jorak that the psychic has detected the Doctor but the sergeant warns him that the woman might be mistaken or, worse, setting a trap. He advises that Grelt uses the humans to catch the Doctor for them and Grelt agrees.

It is dark by the time that the three newcomers reach the village near the castle. They are not welcomed but viewed with fear and mistrust. Strangely, the village spokesman denies any knowledge of ‘demons’. The Doctor tells Oswald that this is because they are afraid of being shunned by people of other villages nearby. Oswald steps forward and tells them that it is too late; he heard the story of the demons in the next village earlier that day. The Doctor joins him and says that he knows what the demons are and that he can help them. This is too much for the crowd who pelt the newcomers with stones. Gisella immediately throws two villagers away from her and disarms a third who is attacking her with a pitchfork. This strength in such an (apparently) young girl is clear proof that she is a witch. The crowd surges forward and overpowers her. They carry her to the pond where she is tied into a ducking stool. The Doctor tries to intervene but he is told that it will be his turn next.

In the castle’s chapel, Sir Howard is reflecting on the promises that the aliens made in return for the castle’s help with their ship. He thinks of the Eternity Crystal as he looks at the statue of his dead wife. Sir Robert Markham, his knight-at-arms enters and tells Sir Howard that Grelt wishes to speak to him. Markham is enraged that Sir Howard is letting the aliens mistreat the castle’s people but is told that the decision has already been made.

Gisella’s ducking is halted by the timely arrival of the local friar, Father Meadows. He argues that enough people have already died without adding a young girl to the tally. The Doctor shows him his psychic paper that seems to say he is a special envoy of the king, investigating demons and witchcraft. The friar tells him that the previous night a star fell to earth amid a sudden lightning storm. Seven men from the village went to investigate but six died instantly. The seventh is lying in the church.

Markham enters the Great Hall of the castle where the Krashoks are reassembling their engines. Blackheart’s servants are staggering about under the weight of heavy equipment. Grelt tells him that more servants are needed from the village including women and children slender enough to crawl inside the Krashok ship’s conduits. He also gives Markham the task of finding the Doctor, whose image he projects onto a wall. Markham refuses at first but when Grelt offers him a powerful gun as a reward Markham accedes.

In the church a young man lies on a makeshift bed, writhing in agony. The Doctor quickly concluded that he was hit by a Martian sonic disruptor that is still resonating in his skeleton. He uses the screwdriver to counter the vibrations and heal the youth. He tells Gisella and Oswald to hunt out some milk and cheese for the youth’s calcium levels but no sooner have they left the church than there is a scream outside. Oswald races back in saying that a dozen armoured men have arrived from the castle. It is Markham and his men. They send more villagers to the castle to labour for the aliens. Ten of the soldiers then set about hunting for the Doctor. Friar Meadows guides the Doctor and Oswald to a secret panel that opens onto a hidden passage. He says that this will lead them to the castle but adds that the passage has not been used for years. The last time it was opened was when a band of outlaws raided the village. The nuns who used to worship there tried to flee down it but the first nun, a novice, entered the passage first and in panic sealed the door behind her.

Gisella is taken along with the villagers. She hides her face behind a shawl so that Grelt will not recognise her. She overhears Grelt telling Jorak to scan for alien technology and is terrified that this will lead the Krashoks straight to her.

The Doctor arrives at a strange cave that smells of roasting meat. Before he can enter he sees a man that Oswald identifies as Blackheart entering. Blackheart shouts for the witch and eventually the old woman arrives. She tells him that by the end of the next day he will be reunited with his dead wife. He complains bitterly at the way the Krashoks are treating his people but she reminds him that the price of love is high. As they separate the Doctor orders Oswald to follow the witch to the surface while he will confront Blackheart. As they part he gives Oswald the sonic screwdriver to pass on to Gisella.

When Markham returns to Grelt he is insulted by the alien for failing to find the Doctor. Grelt sees the fury in the knight’s face and offers him a weapon, a blaster pistol, to use to kill the Doctor when he finds him. Markham immediately turns the gun on Grelt and pulls the trigger. Sadly, Grelt has removed the power pack. He produces a Cybergun of his own and kills Markham.

The Doctor locates Blackheart in the chapel of the castle. He tells the Lord that he knows what the Krashoks are and that he knows they have promised to raise Blackheart’s wife from the dead. He adds that this will never happen; the machine that is supposed to bring her back will just make her a form of living dead. He also tells him what the Krashoks really want the Eternity Crystal for. When a squire enters and tells how Markham was killed by Grelt, Blackheart decides to send the demons back to hell.

Oswald follows the witch for what seems miles through the underground tunnel. He comes out into the open air very close to the alien ship. He sees the villagers carrying the equipment and then he sees the Krashoks for the first time. He backs away and bumps into the witch. In horror he reaches for the screwdriver and switches it on.

Gisella carries some heavy equipment (pretending to struggle with the weight) into the ship. She is directed into a conduit and uses the opportunity to locate a vent. Furtively she manages to usher all of the other children in the ship to safety through this vent. She then connects the power coupling as she has been instructed. Just then she hears Jorak tell Grelt that a sonic device has been located close to the ship.

The Doctor plucks the screwdriver from Oswald’s hand. The witch tells Oswald that it isn’t her he needs to be afraid of, pointing to the aliens. A shot from Grelt’s laser rifle narrowly misses them. As he takes aim again a dozen knights on horseback charge at him. Before the Krashoks can react the Doctor presses his screwdriver to a communicator that Blackheart gave him. The aliens are disabled by the deafening shriek that fills their helmets. The villagers flee while the knights cut down the Krashoks with their swords. Grelt manages to rip his helmet off as Blackheart attacks him and kills the human with a shot from his Cybergun. As Blackheart tumbles from his horse the witch comments that Blackheart has been reunited with his wife as she foretold.