A huge beast is wreaking havoc in a small village. A gang of young men with pitchforks and scythes try to fight it but it merely tramples them to death. Bill Farrow is about to be killed when a rhythmic chirruping whisper seems to halt the monster. Bill sees a young girl, Molly Melton, eight years old, standing in front of the beast, unafraid. Sir Albert Morton appears, holding a small object, the source of the noise. The beast suddenly awakes and turns to run in a different direction. Farrow thinks he is saved until Morton transforms into a squat, hideous demon that reaches for his throat.

When Martha and the Doctor emerge from the TARDIS it is clear that they are not in Berlin and probably not in 1908 either, which was their intended destination. The Doctor had planned to help Minkowski with his address to the Assembly of German Naturalists and Physicians concerning the fourth dimension (space-time). Martha is relieved that they are not back in New York, but appear to be in lush and beautiful countryside. Their conversation is truncated by the sound of a car crash.

They rush to help and discover an Opel double phaeton, which looks like a vintage car to Martha, but is brand new, crashed into a wall. Its owner, Victor Meredith, has a cut hand. While the Doctor performs the introductions and extols the virtues of the car Martha expertly tends to the injury. The Doctor says Martha is an ambassador from Freedonia, though she puts in that she is training to be a doctor. They learn that they are in the Lake District. Meredith tells them that he is joining the hunt for the Beast of Westmorland. Their blank looks cause him to show them a newspaper (September 16th 1909) which shows an artist's impression of a dinosaur-type creature washed up on a lake shore. Meredith adds that it died of a serious head injury, possibly caused by its mate, which has been sighted in the area.

The Doctor starts the damaged car, surreptitiously, with his sonic screwdriver and asks to be taken to see the dead beast. In fact, he insists on driving. On the journey they learn that the dead beast is at Templewell but the base for the hunt is Lord Haleston's Goldspur estate. They drive off without seeing the hunched orange creature in the nearby gorse.

During a very bumpy drive to Templewell Victor informs the time-travellers that a large hunting party is gathering at Goldspur and that some men have even brought their wives along. He says that forty people died in the attack on the village of Kelmore, including Sir Albert Morton who disappeared after running after the monster. He says the king has offered a special medal to whoever kills the second monster.

Templewell is closed off to the public by police but they easily push through. They find the monster on the shore of the lake and are amazed by the huge size, its jaws alone being twice the length of a railway carriage. Most of its head has been ripped away. The Doctor tells Martha that it is a Skarasen: part organic, part metal. He tells her he met one in Loch Ness 'ages ago'. It was made by Zygons and is their cyborg creature.

Lord Haleston is upset. A naturalist at heart, the dead beast has destroyed his orderly view of the natural world. He has been asked by the Prime Minister to conduct a secret government inquiry into the creature. The Doctor's arrival further annoys him. The Doctor claims he is an expert in 'most things, really' and claims friendship with the Prime Minister, Mr Asquith, referring to him as 'HH'. However, his claim that the beast does not originate from Earth (proven by its metallic bone structure) infuriates Haleston. He speculates that the damage to the skull, caused by a sudden impact from within the head, was caused by a hunter from an alien planet with weapons of unspeakable power. He is perturbed by the thought of such weapons on the loose in Westmorland.

Haleston sends them away in disgust at such improbable ideas. As the Doctor drives off he tells Martha about the Zygons he met in Scotland and their plan to take over the world. He explains that their only source of food is a Skarasen's lactic fluid. Suddenly he jams on the brakes to avoid a Zygon that tumbles into the road in front of them. It has a bad wound in the head and injured legs. Before they can speak to it, the Zygon dies and fades away, 'zapped into particles' by the crew of the Zygon ship. Deciding that the alien's injuries must have been caused by something up the hill that it fell from they go to investigate but find only a herd of cows. Martha wonders if the alien hunter killed the Zygon, too.

Victor calls them back to the car. In the gathering darkness they can just make out the shape of a young girl on a hillside, pointing to Toombs' Fell and Wolvernath Mere across the valley. The girl abruptly vanishes into the night. They drive off while the Doctor informs Martha of the Zygons' ability to kidnap people and assume their shape, keeping the captive prisoner so that the body-print technology can be continually refreshed.

Edward Lunn is out at night with his English Mastiff. He is on the trail of the second beast near Wolvernath when he encounters a Zygon. Before he can shoot it a second Zygon attacks him from behind and he runs away. He finds himself trapped on the edge of a precipice over Wolvernath Mere as five Zygons close in. Realising he cannot escape he shoots and kills one of them before leaping over the cliff.

Dropping Victor off at Goldspur it occurs to the Doctor that he and Martha have nowhere to stay. Victor suggests the hunting lodge a half mile away and they decide to try their luck. Mrs Unswick, who runs the lodge, gives them two rooms. She says she has only one other guest. Martha has a quick bath in a freezing cold room and discovers a skirt and blouse in her wardrobe which she puts on to replace the clothes she muddied scrambling across the fields. Mrs Unswick tells her the clothes had belonged to Clara, her maid, who left them behind when she stole the silver a week earlier. She also tells them about Molly Melton, the little girl who has been missing for two weeks. Her apparition has been seen on the hills, pointing visitors away from danger.

Claude Romand enters the room carrying a Pathe film camera. He is a reporter from the News of the Globe in Paris, hoping to film the hunt for the beast. After the Doctor fixes the camera's focus, with a hearty slap, Romand takes them to his room and shows them the film he took of the destruction at the village of Kelmore. The images haunt Martha but her attempts to sleep are interrupted by Victor Meredith banging on the door of the lodge. He asks for Martha to go to Goldspur at once to attend to Edward Lunn who has returned badly injured. After Haleston's doctor could not be found Victor has suggested Martha as an adept healer and been sent to collect her.

The butler, Chivvers, admits them to Lord Haleston's house and they find the lord and Lunn's wife beside the bed of the injured man. Lunn has scratches to the face and neck and a large gash on the head. Mrs Lunn says that Teazel, the dog, also has a large welt on its back. Lunn himself is almost incoherent. He can only remember the little girl pointing to Wolvernath and being attacked in the forest and in the water. However, when Martha suggests an X-ray Lunn refuses in an aggressive manner and demands that the hunt go to Wolvernath. The dog, Teazel, trots in and sniffs at the Doctor's pocket, prompting the Doctor to leave. Outside the room Martha has a quick, conspiratorial, conversation with the Lunn's thirteen year old son, Ian.

The Doctor surmises that the injury the dog sustained was a Zygon sting. He explains the dog's interest in his pocket was caused by an orange growth he pulled from the teeth of the Skarasen and hid in his pocket. It is a homing device for the monster, used by the Zygons to control its movements.

Unable to sleep, Lord Haleston looks onto the fells where a speck of light attracts his interest. Despite the fact that he needs his binoculars he is shocked to see Molly Melton not only pointing at Wolvernath Mere but looking directly at him.

Martha wakes to a disgusting breakfast of almost raw eggs and burnt bacon. The Doctor is absent and in a conversation with Romand she suddenly leaps to some conclusions; Lunn did not want an X-ray because it might show up his alien bone structure and he is a Zygon hiding at the heart of the hunt. She heads off to Goldspur at once.

She meets Ian Lunn exploring the gardens and he shows her a way into the house through the conservatory. Not wanting to be seen uninvited in the house Martha hides in a room, evidently Lord Haleston's study, while Ian scouts ahead. She disturbs a Zygon reading Haleston's diary. It throws the book at her and smashes a window while escaping. Ian returns to find out what the noise was and they are both caught by Nanny Flock, the boy's governess. She is an angry, mean-looking woman who accuses Martha of breaking in. Martha says that there was another intruder before making off for Edward Lunn's bedroom.

The Doctor, meanwhile, has hitched a lift on a milk cart to the TARDIS. He whiles away the time with his sonic screwdriver, tinkering with the Zygon homing device which occasionally comes back to life. Later, as he makes his way across the fields, he notices that the device is attracting some cows and realises it is active again. He shakes it and in the far distance sees a Skarasen staring at him from above the treetops in a forest. He is promptly attacked by a Zygon racing across the field. He overpowers the Zygon and starts a conversation. He tells the alien that he knows something has killed one Skarasen and at least one Zygon and he offers to help. The Zygon merely demands the return of the trilanic activator the Doctor has been tinkering with. The Zygon even says 'please' which startles the Doctor but then launches another attack. The Doctor turns and runs to the TARDIS.

Mrs Lunn prevents Martha from seeing her sleeping husband so Martha leaves the house to find Romand outside in his car, accompanied by the Doctor. The Frenchman drops them at the lodge on his way to trace the hunting party. The Doctor guesses that there could be hundreds of Zygons in the area in the shape of most of the inhabitants (who would still be alive, captive on the Zygon ship). He says the TARDIS scanner showed no recent sign that any spaceship landed (or crashed) and there is no evidence of an alien Skarasen hunter.

He opens the locked door of the Lodge with his screwdriver and they discover Mrs Unswick upstairs watching Romand's film before it is collected by the police. She says she is amazed by the sight of people she knows moving around in pictures. The Doctor joins her and is surprised to see a shot of some extremely large lifting gear in the barn at Sir Albert Morton's residence. When the Doctor asks her how to get to Kelmore to see for himself Mrs Unswick offers to lend him two horses from the stable.

In Edward Lunn's bedroom two Zygons are talking. One is Algor, the other is his commander Brelarn. They are wondering how the humans know who they are. They think the Doctor is dangerous but he may be of value to them. Brelarn orders the field troops to keep him under surveillance. They also discuss Haleston's plans to shoot into Wolvernath Mere to disturb the Skarasen and his plan to use the construction materials at Kelmore to restrain it. Algor comments on how low morale is among the Zygon crew.

On the way to the stables Martha sees a large black carriage which Mrs Unswick says a friend of hers left there. She would have preferred o borrow the carriage but fortunately remembers enough of her riding skills from girlhood to follow the Doctor to Kelmore. On the way the trilanic activator is sparked into life, upsetting the horses. The Doctor explains his plan to trigger it with the sonic screwdriver before using it to subdue the Beast of Westmorland and send it to the Arctic where the Zygons will have to follow.

Ian has been confined to his room but escapes down the ivy on the wall outside when he hears Teazel's frantic barking. He follows the dog to the wall of the estate.

At Kelmore the Doctor is seriously impressed by the size and quantity of the lifting gear in Morton's barn. They meet Victor who has been visiting lady Morton on Lord Haleston's behalf to arrange the use of the gear to capture the Skarasen. The Doctor is worried that this plan will fail and kill dozens of people so he arranges for Victor to take Martha over to Wolvernath on the pretext of her needing to see Romand (but really he wants her to disrupt the hunt while he uses his time to fix the homing device.)

Martha and Victor's journey is interrupted when they see a black horse-drawn carriage - the one she saw earlier at the lodge - being pursued across a field by a group of Zygons. The carriage crashes into the hedge beside them and then tips over. Two Zygons are killed but the other three ignore them and try to get the carriage doors open. As if from nowhere Teazel appears and rips the throat of one. The other two disappear into a group of cows. Martha looks into the carriage and sees a large organic box.

Ian arrives saying he has followed Teazel. Martha tells Victor and that the three corpses are of aliens called Zygons. Ian takes something that looks like a dagger (made from vegetable matter) from the hand of one of them before all three vanish. Teazel bounds away. Victor realises they need to get help for the injured horse and the trio make for the Lodge where there is a telephone.

In the Zygon control room analyst Taro and her assistant Felic have watched this scene with dismay. Three lives lost through greed. On another screen Felic sees the Doctor and says he is playing right into their hands.

The Doctor is struggling with the homing device in a field when he sees Molly up the hill. As he approaches her he is knocked unconscious by a Zygon who crept up behind him with a heavy stone.

At the Lodge Martha discovers that the phone wire has been ripped from the wall and Nanny Flock is there waiting for Ian. She accuses the governess of being a Zygon, at which point Mrs Unswick appears and knocks Nanny Flock out with a brass bed-warmer. Martha's relief is only brief when Mrs Unswick reveals herself to be a Zygon and takes hold of her throat.

On the Zygon ship the Doctor is shown Martha's capture on a screen. Taro asks him what his plan is for the trilanic activator and he tells her. She in turn explains that the Zygon ship crashed into the Lakes and that a rescue party will take centuries to arrive. Brelarn orders the Doctor to complete the repairs on the device at which the Doctor comes to several conclusions. The Zygons have lost control of the Skarasen and want the humans to recapture it for them; the Zygons ordered delivery of the machinery using an impersonated Morton; molly is a Zygon pointing out the whereabouts of the Skarasen. What he can't understand is why the Zygons are using humans instead of taking their weapons and machines to do it themselves. Brelarn gives him two hours to repair the device or Martha will be killed.

After being locked into a room with Ian and Victor, Martha examines the object that the boy took from the dead Zygon's hand. She guesses that it is a tube containing liquid food. She cannot understand why Zygons would be stealing food from a Zygon carriage but decides to squeeze some of the 'food 'under the door to see if her captor wants it back. The response is immediate, the door crashes open and the Zygon demands the 'ration'. Martha concludes that it is Skarasen milk and that with one beast dead there isn't enough to go around. That was why the Zygons were attacking the carriage. She throws the ration into the corner, the Zygon leaps after it and the three humans run out of the door and lock it. They find Nanny Flock just waking up but the Zygon bursts the door off its hinges and the heavy oak crashes into her, flooring her again.

In the Zygon laboratory Felic tells the Doctor that the stellar catastrophe tat downed their ship also affected the brain-computer interfaces of the Skarasens. Attempts to rectify the problem on one inflamed the tissue until its head blew open. The other is stupefied in the lake awaiting therapy. The Zygons are too weak, physically, through lack of food to catch it. They are conning the humans into doing it for them. This is the real sting of the Zygons. The doctor says that they should have asked the humans for their help instead of kidnapping and murdering them. Felic says that this is only part of Brelarn's plan to take control of the Earth.

Several versions of Molly Melton enter the laboratory. They are Brelarn's own children but they are too weak to continue in the fields. They are put into chambers with amber resin to hold them in stasis until food arrives. The Doctor completes his repairs on the homing device and demands that he be allowed to use it as only he understands the screwdriver that powers it.

Martha, Victor and Ian hide in the stables because the stink from within might mask their scent. In the straw Martha discovers the corpse of Clara with a knife between her shoulder blades. Once the Zygon leaves the area they escape in Victor's car. On the road they meet Romand in his vehicle. Martha sends the other two off to Goldspur while she goes with the Frenchman to Wolvernath. She learns, too, that the king is on his way.

On the road they encounter the Skarasen but it is quite docile, being led by the Doctor and followed by the hunting party. The Doctor is surprised to see Martha and perplexed when she tells of Clara's death as he does not seem to remember her. The Doctor accepts a ride in Romand's car to hasten the journey to Templewell, though even that has to be slow as the engine vibrations interfere with the signal. Romand asks Haleston if he can film the king's inspection of the beast or at least the ceremonial dinner afterwards. Haleston agrees to the latter.

At Goldspur Ian meets his mother returning from sending a telegram from Kendal. Victor and he search the grounds for Zygons. There are none but they find a telegram sent by Lord Haleston inviting the Prime Minister and the cabinet to discuss the nature of the Beast. Victor notes that the telegram was sent that day, which is odd considering Haleston's presence at Wolvernath all day. Ian wonders if the telegram fell from his mother's basket when she dropped it as he greeted her.

Their conversation is interrupted when Mrs Lunn asks Victor to find Chivvers while she takes Ian to his father. In the bedroom Ian's parents reveal themselves as Zygons. Victor finds Chivvers, dead, and is attacked by Teazel who transforms into Brelarn. Brelarn says the guise of a dog gives him swiftness and access to conversations that would otherwise be private. He says he killed Zygons stealing food from the carriage because the penalty for mutiny is death.

At Templewell the Doctor lays the beast beside its dead companion and men begin to secure it with heavy nets and chains. He takes Martha to a hut to discuss their next plans and to her horror transforms into Felic. He is attacking her when Howard Chisholm bursts in and beats him unconscious with a spade. They leave the hut as Romand drives off with Lord Haleston.

The Doctor wakes up after the body-print connection to Felic is broken. The real molly Melton is beside him and she shows him a long corridor full of humans in chambers, unconscious while Zygons impersonate them.

Martha and a party of men arrive at Goldspur to search for Zygons. They are surrounded by a party of cows. Romand also drives into a herd of cows. The cows, and Romand, transform into Zygons. Lord Haleston scrambles to escape from them and knocks the film camera from the car. It breaks open and a pistol falls out. At Goldspur the Zygons show Ian Lunn to the men causing them to lay down their weapons.

Haleston is marched through the forest by his captors. Their journey is halted when the Doctor speaks to them from the trees. He says the space ship was almost unoccupied and he simply walked out. He wonders what would cause all the Zygons to leave the ship. Haleston suggests it is the king's visit. The Doctor concludes that they intended to execute the monarch and use his funeral to capture and impersonate all the heads of Europe who would attend his funeral.

Suddenly a herd of cows, bulls and horses race out of the forest, trampling through the Zygons. Haleston runs to safety in the trees. He meets Romand, assuming him to be a Zygon, but realises that it is the real journalist when he pretends to be a Zygon pretending to be him to fool another Zygon.

The Doctor is busy sending the newly awoken humans to the Manor's wine cellars while he tries to end the affair. He tells Haleston that the ultimate end of the Zygon plan was to change the climate of the planet to suit  themselves. He leads them to the Lodge in his next bid to save the world.

The men of the house are prisoners elsewhere while the ladies sit in the Goldspur drawing room playing cards and drinking tea so that their weary Zygon captors can learn the rules of social etiquette. Martha overhears that Taro is close to finishing her work on the Skarasen and resolves to escape. Ian tells her that a lock on one of the windows is faulty.

At the Lodge the Doctor is attacked by a Zygon. It throws off Haleston and Romand but is knocked out by Nanny Flock who has been hiding within. She tells the Doctor that the Zygon kept babbling to itself, though he suspects it was communicating with others. He finds it was preparing the last of the rations to send to Goldspur. Suddenly a party bearing lights turns into the driveway.

After Ian and Victor cause a diversion for her Martha escapes from the drawing room and runs into the Doctor. They catch up on each other's news then go to the front of the house. Brelarn is leading about thirty Zygons out of the house, each with a human prisoner. The doctor offers Brelarn one last chance to return to his ship but the Zygon warlord repeats his intention to take over the planet. The doctor sends a distress flare into the sky before throwing Zygon rations to the guards. The aliens, near starvation, begin to release their hostages and fight over the food. The Doctor, Martha, Ian and Victor herd the humans to safety as the cavalry - the Eighth King's Royal Hussars - arrive in response to the flare and slaughter the Zygons. The Doctor says he shut down the body-print controls on the ship so that the Zygons cannot hide in the shape of others. Despite the celebrations of Romand over the cavalry's victory the Doctor is not in the mood for enjoyment. Instead he heads off to Templewell.

The Doctor takes Martha in victor's car to Templewell but when they see the police guard lying in the road he diverts down an embankment to the lake shore. They are attacked by Teazel but the mastiff transforms into Brelarn as the body-print fails. Evading him they reach the Skarasen's head as its eye opens. The Doctor fights Brelarn while Martha rushes off to the hut to get the Doctor's screwdriver and the activator.  After knocking the Zygon out the Doctor pretends to Taro that he is Felic. She complains that the engine of the car, still running, is interfering with her control of the monster. Suddenly the monster roars and Brelarn reappears. Taro leaps on the Doctor.

Martha opens the shed and reaches for the two devices. Felic is not dead, however, and grabs the activator, ripping out the Doctor's repairs. This is his last act, and as he dies she takes the activator and runs.

Taro stings the Doctor with enough force to kill a human. He isn't one, and fights her off. As she goes to switch off the engine of the car Brelarn pursues the Doctor. Martha arrives to give him the activator and Romand reveals himself by shooting at the Zygon Warlord. With the Zygon pinned down the Doctor gets Martha and the Frenchman to start the engines of every machine at the site. The Skarasen rises in anger at the interference to its control matrix and lunges at the crane, causing the humans to dive for cover but allowing Brelarn to pick up the activator. The doctor tells him to throw it away but Brelarn thinks this is a ruse to trick him until the Skarasen swallows both him and the device.

Noticing its dead companion, as if for the first time, the monster backs into the lake, then drags the corpse of the other with it. The Doctor says there is an underground channel to the Irish Sea, through which the beast can escape.

He then goes to find Taro who was wounded by a shot from Romand earlier. She thinks the Doctor intends to kill her but he tells her to return to the ship and call back any surviving Zygons. They can rejoin the children and with the surviving power (he checked) make one last flight before retuning to the amber to sleep until rescue ships arrive.