In the fields outside Worcester, Brother Neame is knocked from his horse and falls unconscious. When he recovers his horse is dead. He limps on a sprained ankle trying to get back to the city. Then he is knocked to the floor again and looks up into the face of the Devil, lunging in for the kill.

Three months later, the TARDIS materialises in a rat-infested alley. The Doctor steps out and smells the fresh air of pre-industrial England. It is 1139 and Worcester has barely two thousand inhabitants, yet even so the snowy streets are deserted. Night has just fallen but all the doors are closed. He knocks on an inn door but there is no response.

Inside the inn the keeper, John Garrud, makes his way to the door. The Doctor’s repeated hammering has forced him downstairs. He opens the door a crack and tells the Doctor to go away. The sound of heavy footsteps on the street changes his mind and he pulls the Doctor inside. His wife creeps down the stairs and is displeased to see the stranger. John asks her to make up a room for the guest but Gertrude wants nothing to do with it.

On the street outside armed men go clanking past. John Garrud asks why the Doctor is out after curfew. The Doctor gives a non-committal answer, saying he didn’t want to be travelling at night. Garrud agrees, what with the Huntsman being about. Checking through his memories the Doctor recalls the legend of the Devil’s Huntsman hunting lost souls on the Clent Hills. Garrud explains that since the death of Brother Neame there have been many other local people who have gone missing from the outlying villages.

Several hours later, while his hosts sleep, a scream is heard in the sky, perhaps a large bird, and then another scream – human this time – comes from closer at hand. The Doctor throws on his coat and races into the streets.

In a small yard he finds human remains and a Krillitane.

Emily pulls her hood back and scans the buildings through her binoculars. She has spent a week placing Bio-locator Pods around the city, but since they were activated she has seen nothing for three nights. A snowstorm has reduced visibility from her vantage point in an oak tree. A power surge in one of the Pods fades away. Grimacing, she decides to find the pod and check it out. After dodging the heavily armed patrols she has found and checked fifteen pods, all working. Annoyingly, a blue shed is standing where the sixteenth pod should be. She touches the shed, surprised at its vibration, when a scram rings out. She quickly climbs onto the low roofs and makes her way to a yard where she sees a hideous beast feeding on a dead man. She presses record on her binoculars and zooms in. just then a man, the Doctor, arrives on the scene.

As soon as it has seen the Doctor, the Krillitane flies away. The tramp of heavy feet tells him that it was the approaching patrol that drove it off. He ruminates on the Krillitane’s appearance. Eight hundred years later they would have shorter necks and a shorter tail without a forked tip. A glint of metal catches his eye and he picks up a small device with a red button. He has barely had chance to pocket it when the soldiers arrive. They gag the Doctor and tie his hands behind his back.

Captain Darke, a career soldier, looks at the Doctor and proclaims that he has the eyes of a killer. He tells his men that they have caught the Huntsman. They lead him away and Emily follows across the rooftops.

The patrol makes its way through Worcester. Doors open as they pass and onlookers step out to form a sizeable crowd. Darke pushes the crowd back and orders his five men to present arms. Emily realises that the Doctor is in danger. She strips off her all-weather jacket to reveal medieval clothes. She drops to the street and forces her way through the crowd until she can kiss the Doctor and demand of the soldiers some answer for the way they are treating her husband. This buys enough time for a larger party of soldiers to arrive and drive the crowd away. Darke orders the people to return home. Muttering, they do so.

Emily tells Darke that she heard a scream and her husband went out to investigate. The Doctor agrees with her and Darke begins to doubt his own judgment. He orders two men to take the married couple to their rooms and says he will take their statement at daybreak. As they are led away the Doctor looks at the pretty girl beside him and sees that her clothes are made of artificial fibre. She is clearly another visitor to Earth.

The Krillitane, Toeclaw, flies in panic from the city. She knows that someone will soon push the button that will end her life and is trying to get out of range. Her belly is warm from the thrill of killing the man but her strength is failing. A shock of pain sends her tumbling to the ground.

She wakes in a copse of trees and finds that they have cushioned her fall. Another jolt of energy shoots through her spine. She remembers the inhibitor in her muscle at the nape of her neck. Twisting awkwardly she finds the lump under her skin and rips it out before casting it away in triumph. She vows to return to her brothers and sisters and release them from their bondage.

Captain Darke finishes his examination of the corpse in the yard and recognises that no human could have caused such damage. A messenger arrives and tells him to report to the sheriff. Darke makes his way to the Castle. He thinks of the news that Matilda and her followers are now in England and looks at the defences that the new Sheriff has erected. Even with the news of Matilda, Darke considers the defences as too strong. He wonders if the new man is paranoid.

The Sheriff is a small man with few social skills. He is annoyed that a citizen broke the curfew and orders that nobody may enter or leave the city without his permission. He finishes by demanding that the Doctor is brought to him.

The Doctor, meanwhile, is in a sparsely furnished room with his new ‘wife’. He introduces himself to her and she gives her name as Emily Parr and thinks that the Doctor is a bounty hunter sent after her by her father. She is disinclined to believe him, with Earth being so far from the nearest trade routes. The Doctor tells her to leave for her own safety but Emily persists in saying that she is just hiding from her family. The Doctor tells her that the Krillitane are not native to Earth and shows her the device he found by the corpse. She is puzzled, it is clearly a tracking device and the man who had it was another off-worlder. She also begins to revise her opinion of the Doctor when he gives her a potted history of his dealings with the Krillitane and reveals that they are shape-shifters. Emily makes an intuitive guess that the Sheriff is a Krillitane and that the Castle is home to a Krillitane horde.

Darke is astonished to find the Doctor gone when he arrives at the room. The guard, Butcher, allowed him out after reading a ‘parchment’ (though Butcher later confesses to being illiterate). Glancing out of the window he sees Emily crossing the rooftops and is about to set off in pursuit when a creaking floorboard stops him. Under the board he finds a lot of unfamiliar (futuristic) equipment, including a gun.

The Doctor and Emily make their way to the Castle when they are intercepted by Butcher and his partner, Hewse. To atone for his mistake, Butcher tries to arrest the Doctor again but the psychic paper bamboozles him once more. Not only does he let them go but Butcher and Hewse hand over their armour and weapons.

The two escapees use their disguise to get into the Castle grounds before dumping the armour. They begin by infiltrating the corridors, heading towards the Sheriff’s rooms, when Darke catches them. He is holding the rifle and the Doctor assumes it was provided by the Krillitanes until Darke suggests that he knows less about his ‘wife’ than he ought. Somehow, Darke begins to trust the Doctor and listens when he is told that there are monsters from beyond the stars loose in the city. While sceptical, Darke agrees with the Doctor that there is something unusual about the Sheriff and agrees to take the Doctor to him.

Strangely, the Sheriff is immobile in the room when they arrive, his face expressionless. The Doctor examines him quickly and assures Emily that the man is not a Krillitane but his frontal lobe appears to have been switched off. As dawn breaks outside the Doctor becomes aware that the Sheriff is staring at the cathedral and realises that the device controlling him must be located there. The Doctor takes the rifle from Darke and disarms it, then instructs the soldier to stay with the Sheriff while he and Emily investigate the cathedral. They start by running up the tower steps to stand at the top of the Castle and looking across at the cathedral. Through the rifle’s telescopic sight the Doctor sees a neural transmitter near a window. The only thing that puzzles the Doctor is that none of it seems like the work of the Krillitanes. A priest emerges from the Cathedral and Emily takes a photograph. She is astonished when a message flashes up telling her that she has positively identified her target. She knows that this man is Lozla Nataniel Henk. Abruptly, she turns away from the Doctor and says she must leave the planet at once.

Henk has been having a bad night: Lynch allowed Toeclaw to get too close and was killed, and then Toeclaw herself had to be exterminated. Now her corpse must be retrieved before the locals found it. Branlo, a technician, has also reported recording an energy spike near the time of Lynch’s death. There are also heat readings of a non-terran with two hearts near the corpse. Henk is furious that someone he hasn’t invited might be on the planet. He makes his way through a locked door and down a flight of steps to speak to the blue-skinned Velurian, Belima Febron. He needs her to make a presentation when the delegates arrive in a few hours.

The sheriff, awake again, orders the Doctor to be brought to him immediately. He is, of course, unaware that the Doctor has returned to the Castle and is eating soup in the main hall. Darke, however, is equally unaware that the Sheriff is merely a puppet, controlled by Henk through a neural relay. He does know that nobody has spoken to the Sheriff and yet his superior spoke of new intelligence and so the soldier is speculating that the Sheriff is not acting independently. When Darke gets back to the hall the Doctor has gone. Running out into the snow he sees the Doctor striding towards the Cathedral.

Emily steps into a barn, a holographic projection to disguise her ship. Once aboard she uploads the photograph of Henk to her employers. When payment is confirmed in her bank she will leave Earth forever.

Inside the Cathedral the Doctor encounters Bishop Henk. He tells him that he is cataloguing religious items for King Stephen. The Bishop dutifully gives the Doctor a tour of the Cathedral, during which he reveals that he has only a limited grasp of the true nature of medieval life. The Doctor is convinced that Henk is neither from Earth nor is he a Krillitane. Their conversation is interrupted by Branlo who draws Henk aside to tell him that the double-hearted alien is in the Cathedral.

The Doctor realises he has been rumbled and makes for the nearest door, picking the lock easily. In an antechamber to the crypt he sees a bank of monitors. It gives him a view of the contents of the crypt. Intrigued, he makes his way inside for a closer look. He finds a lab-coated woman walking among sleeping Krillitanes. These monsters are hanging from the ceiling, chained and connected by tubes and wires to hidden machinery. Before he can engage the woman in proper conversation Henk arrives and asks what the Doctor thinks of his Krillitane farm. He says they are extracting Krillitane oil. Just as he is about to point out how unethical this is, the Doctor is injected in the neck by Febron and falls unconscious.

Emily wakes to the sound of an alarm signalling a message from her employer. It says that agents will be arriving within one day and advises her to leave beforehand. She preps for immediate launch until a pang of conscience about the Doctor makes her power down the engines. She heads into the late afternoon, heavy clouds signalling more snow is on the way. She trips over a mound of snow, uncovering a still-warm but badly mutilated female corpse. The ridge of cartilage across the nose tells her this was a Calabrian woman. Three monks pounce on her and slap her to the ground. One scans her and announces that she isn’t Toeclaw. He says the Krillitane is still out there. Just then a grey shape drops onto them, killing two of the monks. Emily grabs a discarded blaster and tries to escape. Another monk’s body drops into the trees near her and spins around. Another Calabrian woman is approaching across the snow. She asks if Emily is one of Henk’s team. Emily replies that Henk is a monster and the Calabrian asks if Emily would like to help her kill him.

The Doct0or wakes up tied to a chair in the antechamber to the crypt. Febron is working in the room so the Doctor asks her some questions. She says she is a scientist and that Henk is her financial backer, not her boss. She says she is working on a beneficial agricultural project and they are using Earth because of its lack of technology and its ready supply of meat. She says that allowing the Krillitane out to hunt for their own food improves the quality of the oil she is extracting from them but refuses to say what she is using it for. After she leaves a monk enters to guard the Doctor but immediately removes his hood to reveal his face. He is Captain Darke.

Henk is awaiting the arrival of his delegates when Branlo tells him that the Calabrians will be late. They have landed but need to recalibrate their instruments before leaving the ship.

Emily has realised that the Calabrian woman she is talking to is the Krillitane that wiped out Henk’s men. The Krillitane introduces herself as Toch’Lu of the Krillitane Horde. Warily, Emily agrees to help her get close enough to Henk to kill him. They walk through the countryside to the Calabrian ship (the crew is a pile of bones) while Emily says she needs to get back to the city to find a friend. Emily is kitted out in a security guard’s coverall and helmet. She hides her pistol inside her armoured padding.

The Doctor uses the monitors to watch the arrival of the Siilutrax delegation at the Cathedral he then looks at the sedated Krillitanes, counting thirteen of them. He and Darke enter the crypt and the Doctor approaches one of the Krillitane, Broken Wing, whose sedatives he has turned off. As Broken Wing regains consciousness the Doctor tells him that they have a common enemy. Broken Wing says he needs to avenge the death of the Brood Mother Toeclaw, but the Doctor says he thinks she may still be alive. He asks how the Krillitane came to be in this situation. Broken Wing says that his Brood fled their homeworld to find a quiet planet in a region where their species was unknown. One of their young fell ill so they traced Belima Febron because of her reputation. However, Febron and Henk double-crossed them and have been milking them for their oil since then. The Doctor offers to help them escape in exchange for a promise to leave the planet. He then restarts the sedation. As his senses cloud again, Broken Wing plots his revenge on the Doctor for not releasing him immediately. Meanwhile, the Doctor removes a vial of Krillitane oil from the machinery and decants it into a test tube which he places in his pocket. He says that this is the distilled enzyme that controls the Krillitane’s evolution. It is lethal to the creatures if it touches them but gives them the power to alter their bodies within a generation.

The Chapter House is a huge, circular room. The delegates are ushered into appointed areas: the tall, grey Siilutrax, three Octulans in an enclosed bubble, the imposters disguised as Calabrians. The Doctor watches the monitors as Henk begins his presentation with a film about evolution. Henk steps into the spotlight to begin his spiel but the camera shifts away from him to the Calabrians and the Doctor, wondering why someone is so interested in the audience, accesses the computers. He realises that the Calabrians are not who they are supposed to be.

Henk’s presentation is building to a climax. A screen behind him becomes transparent and reveals a pen holding a giant beast, a superannuated Krillitane. Henk tells the delegates that it was created by Febron and announces it as the Krillitane Storm. At this sight Toch’Lu transforms into her real shape and howls. Emily throws off her helmet and the Doctor tells Darke to follow him. Toch’Lu launches herself at Henk on his podium but he stares impassively as she smashes into an invisible force field. Emily forces her way through a scramble of delegates towards the nave. She is pursued by monks with guns but the Doctor drags her through a door which Darke slams shut. They brace the door with a wooden bar.

Emily says she had to help the Krillitane after she saved her life but she intended to return to warn the Doctor about something. Before she can go on another door bursts open and they run but they are soon surrounded by armed monks. Henk and Branlo walk across to them and Darke’s unconscious body is dropped at their feet. Emily produces her pistol and points it at Henk’s face. Henk seems unconcerned. He notes that she is Ertrari in origin and wonders if her appearance has anything to do with the Ertrari bounty hunter he had tortured and killed a month earlier. Emily spits out that it was her father. Henk adds as an aside that her father made a pleasant meal for the Krillitane. Emily squeezes the trigger but Henk is too fast. He wrenches the gun from her and punches her on the cheek. Turning on his heel, Henk says that she won’t be alive long enough for the bruise to show.

In the crypt, the stunned Toch’Lu is examined by Febron who then fits a collar to transform her back into her Calabrian shape to make her easier to manage. Henk orders her to be locked up with the Doctor and his friends.

Between Saturn and Jupiter a starship jumps out of hyperspace and turns towards the Earth.

Darke is awake again. He sees the unconscious Krillitane, dumped on the floor, and the Doctor talking to Emily. She is telling him how her father had died tracking Henk and she dropped out of college to carry on the job he was on. When she was set to leave the planet her employers told her it would be best if she left the planet so she came back to warn the Doctor. She doesn’t know who the employers are, however. The Doctor wonders whether it wasn’t actually Henk that was being sought and looks at the Krillitane.

Order has been restored in the Chapter House and Henk resumes his presentation. He blames the interruption on internal Calabrian protests and introduces Febron. She explains how Krillitane oil can be used to help any race evolve as it chooses. As soon as she is finished she goes back to the crypt. She is alarmed to find the vial of the Krillitane enzyme empty. She also sees that Broken Wing’s sedatives are nowhere near enough to keep him asleep. A noise alarms her and before she can get back to the door she is cut off. Broken Wing is about to begin his revenge.

The Doctor tells Toch’Lu that he knows she is only pretending to be asleep. When he says that a ship is due to arrive from off-world she gives him her full attention. She tells him that the ship is not after Henk but has come from the Esteemed Father to punish her and her rebel Brood. The Doctor says he will help her if she promises to leave. When she agrees he takes the collar from her neck.

The Krillitane smashes the door down and pounces on the monk outside but the Doctor restrains her before she can kill him. They hurry to the crypt and the Doctor stops the flow of sedatives to the Brood. As he does so he is attacked by Broken Wing but Toch’Lu blocks the assault. Broken Wing is delighted to see her alive but furious that she denied him his revenge. Toch’Lu assents to the Doctor’s demand that they leave the planet.

Henk is so annoyed the prisoners have escaped that he kills the man on the security monitors with his blaster. He is all too aware that the crypt may be in the hands of the Krillitane, and Febron too, and is uncertain what to do next. Deciding that he has a large supply of Krillitane oil and Febron’s notes he concludes it would be best to take a few Krillitane and escape. He orders the crypt to be opened and then follows his men in. they are greeted by thick clouds of smoke where the Doctor has set the research station on fire.

Inside the crypt the Doctor has woken all of the Krillitane and Toch’Lu has found a secret stairway. On the way up the stairs the Doctor hands Darke the tube of Krillitane extract and tells him of its power. The Krillitane reach a balcony above the nave and Broken Wing leads four of them away from the main group where they take to the air. Henk shoots one down and retreats to the cloisters.

The Doctor and the others make it out through a window onto the snow-covered roof. A storm is swirling around them. The Doctor, Emily and Darke are lifted by their companions and flown down to the lawns. No sooner have they landed when a deep-space frigate drops through the clouds and dozens of Krillitane drop onto the tower.

The shockwave of the ship’s arrival smashes all of the Cathedral’s windows, interrupting the battle between the monks and the Krillitanes. Henk watches the ship land between him and the boathouse, his intended escape route. He returns to the podium in the Chapter House and reactivates the force field. Broken Wing follows him until he sees a larger, wingless Krillitane enter. The Doctor leads his band of followers to shelter in some woodland outside the city but Darke insists on returning to defend the people. Toch’Lu sends her daughters to the Calabrian ship but says she cannot go with them or they will be hunted down. She, the Doctor and Emily accompany Darke to the city gates. Darke instructs his men to help evacuate the city while he leads a party to set up a defensive perimeter.

The Esteemed Father and Broken Wing hurl insults at each other until Henk intervenes. He says he will tell the newcomers where Toch’Lu is if he is granted safe passage to his ship. Outside, the Krillitanes and the human soldiers engage in a fierce battle and the Doctor, Emily and Toch’Lu have found Henk’s hover skiff in the boathouse, laden with barrels of oil. The Doctor says he will not use the oil if Toch’Lu can persuade the new arrivals to leave the planet. She races off to the Cathedral and finds herself confronting the Esteemed Father.

She says that she is honored he has come after her in person. He says she is not fit to call herself a Krillitane and she responds that he is not fit to rule. The Doctor steps in to warn the Esteemed Father to leave Earth. Broken Wing uses this distraction to slay one of the Father’s guards and Toch’Lu attacks the Father himself. Henk fires a shot at the wall behind the Doctor and runs out of the door. The wall crumbles and suddenly the Krillitane Storm stalks out of its prison. The Doctor flees the Cathedral and locks the door, albeit uselessly he reckons. Behind him Toch’Lu and Broken Wing pick up the Father and lift him to the top of the Cathedral. The Father has no wings, Broken Wing cannot fly alone and Toch’Lu is injured and exhausted. With a determined and resigned look at each other the two younger Krillitane plunge with their leader to the ground.

Henk has captured Emily and takes her to the boathouse and onto his skiff. The Doctor arrives just in time to leap aboard as the skiff accelerates down the river. There is a brief struggle which takes Henk from the controls. The skiff slows enough for Emily to gain her balance and smash Henk’s head with a maintenance clamp. The Doctor tells Emily that the Krillitane will never leave Earth now that they have found it. He needs to drive them back to their ship. He uses his screwdriver to adjust the skiff’s circuits then launches it into the air.

The Krillitane Storm steps out of the Cathedral and faces six men, including Darke. They try to hack at the creature’s legs to bring it down. The Storm reaches for him and sweeps him up towards its mouth. In his last moments of consciousness he remembers the test tube and flings it into the Storm’s mouth. It drops Darke into the snow, convulses, whimpers and dies.

Wounded men and civilians make their escape towards the walls. Butcher, Garrud and his wife Gertrude are trapped, however, by a hungry Krillitane. Just as their deaths seem certain Krillitane oil rains down from the airborne skiff. The beast explodes and the others around it flee.

Having driven the Krillitane back to their ship the Doctor lands beside the Cathedral. He finds the corpses of Toch’Lu and the Esteemed Father and the badly wounded Broken Wing. The Doctor offers his condolences to Broken Wing for the death of his mate. The Krillitane says that he shall return home to challenge for the leadership of his people.