The Doctor receives a distress call from the Brigadier calling him to Scotland, and while he’s searching the TARDIS wardrobe for suitable clothing, the TARDIS materialises in a forest in midwinter. Harry emerges to explore, but the TARDIS has been drawn off course, and it corrects itself and resumes its journey, leaving Harry behind. Trying to return for him, the Doctor materialises in the same forest in what appears to be spring; however, when he and Sarah emerge they find that, though the woods are blooming with life, the air is still as chill as winter. Searching for some indication of what’s happened to Harry, they head for the nearest church, where they find a timetable of services implying that this is December... and a gravestone indicating that Harry Sullivan died on 28 November 1936.

Once the shock has subsided, the Doctor and Sarah take a closer look at the graves and find several other stones with the same date, including one for Lady Hester Stanton and one marked only “Godric.” They seek further information at the local pub, but the villagers refuse to discuss those terrible events with outsiders; however, the grieving father of the late Rose Perry directs them to “the Leffy house,” a manor just outside town. The manor’s only remaining occupant is the butler, Trelawney, who knows little of what happened that night. He does tell them that he learned of Harry’s death from Emmeline Neuberger, a cousin of the family who was visiting from Germany -- and that the strange events began shortly after another man, also called “the Doctor,” came to town. This other Doctor has moved away, leaving no forwarding address; Emmeline has moved to London; and the only other survivor, George Stanton, has been committed to St Sebastian’s Home for the Insane. The Doctor decides to track down Emmeline in London while Sarah interviews George.

Back in November, the bewildered Harry is waiting in vain for the TARDIS to return for him when he hears a woman scream in the distance. He investigates to find that a young woman has been killed violently in a forest clearing, her blood soaking into the earth. He is then surrounded by a mob of frightened farmers who take one look at his thick sideburns and accuse him of being the wolf-man who’s been slaughtering their sheep. Fortunately, Harry is saved from lynching by a stranger whom the villagers seem to respect. The stranger identifies the dead woman as Miss Lucinda Ryan, and asks Harry to accompany him to the Leffy house to break the bad news to her fiancé, George Stanton. Harry is shocked when the stranger, a young man with long brown hair and a velvet coat, introduces himself as “the Doctor.” Harry knows about regeneration, but the stranger doesn’t respond when Harry drops hints about the TARDIS and Skaro, so Harry concludes that this must not be the same man he knows.

At the Leffy house, George is nowhere to be seen, but the velvet-clad Doctor introduces Harry to George’s mother, Lady Hester (née Leffy), and George’s German cousin, Emmeline Neuberger. He claims that Harry is a naval lieutenant who missed his ship while trying to save Miss Ryan, and Lady Hester offers to put Harry up in the guest quarters until he can be on his way. Harry thus spends the night at the manor while the Doctor returns to his cottage.

The next morning, the shaken George Stanton bursts back into the manor, claiming to have been attacked by a tree. He’d gone out to help hunt the wolf which had been attacking the farmers’ sheep, and while the farmers were distracted by a commotion in the forest (which George does not yet realise was the discovery of his fiancée’s body), George spotted the wolf and chased it into the forest. While there, he heard an unearthly female scream, and noticed that the fires which the farmers had lit to scare off the wolf were burning out of control and threatening the forest. George put out the fires, but then got lost in the woods, unable to see his way as the leaves seemed to be growing thicker above his head, blotting out the moon. He fired his rifle into the air to attract attention, but hit the trees -- which attacked him, knocking him out until this morning.

As George tells his story, the maid Jane begins to scream, and Harry investigates to find the velvet-clad Doctor watching as the plants in the manor garden visibly grow, fruit ripening and falling from the trees as if the land has come to life. The Doctor theorises that Lucinda was killed as a blood sacrifice by someone who’s using magic to tap the elemental forces of the land, but when he and the sceptical Harry return to the clearing where her body was found, they find that it’s transformed into an impenetrable thicket. A dazed young man named Godric then appears in the woods, dressed in medieval clothing and claiming that Arthur is the King of England. He has no memory of how he came to be here; all he remembers is finding a spring of cool water and then experiencing an intense sensation of peace.

The Doctor takes Harry and Godric back to his cottage for lunch while he plots their next move. Driven by curiosity, Harry snoops around the cottage and finds letters from the editor of Astounding Stories magazine, rejecting the Doctor’s imaginative but undisciplined tales. Harry also finds a tall blue box, similar in size and shape to the TARDIS and connected to a mass of electronic equipment; however, the box is just an empty wooden cabinet, and when the Doctor finds Harry in his laboratory, Harry is too embarrassed to ask him about it.

December: Sarah walks the Fourth Doctor to the train station, where they find that the wild growth they’ve witnessed is confined to the immediate area of the village. They also find a newspaper bearing a date of 11 December; only a fortnight has passed since Harry’s apparent death. The Doctor sets off for London to look for Emmeline, only to find that she’s been taken prisoner by government scientists who have chained her with silver and are conducting cruel experiments upon her -- for Emmeline is a werewolf, and the government wants to create werewolf soldiers for the war they fear is coming. Disgusted by the scientists’ cruelty, the Doctor helps Emmeline to escape, and admits that he knows how to force her to change but chose not to tell the scientists. She decides to trust him, even though the last time she trusted a man called “the Doctor,” he inadvertently betrayed her by delivering her into the hands of people he incorrectly trusted to treat her well.

On their way back to the village, Emmeline tells the Doctor that the werewolves of Germany were confined to camps in 1933. After a year of torture and starvation, a man came to Emmeline’s camp, gave the werewolves food to build up their strength, and offered them a chance to serve their country. By this time, they would agree to anything. On the second night after the full moon, when they were still wolflike but the beast was not out of control, the werewolves were taken to a hotel in Wiessee and set upon a band of “traitors” -- and any innocent bystanders who witnessed the slaughter. Fortunately, Emmeline’s victim was wearing a silver necklace, and when she injured herself biting it, he struck her down with a silver candlestick and fled. She passed out beneath the bed and was missed in the ensuing confusion, and the next day she fled from the town. After several months of living wild in the forest, she remembered having family in England, and managed to escape from Germany and find her way to the Leffy house, where Lady Hester took her in. But the British government wouldn’t let Emmeline remain in England, and as she searched vainly for a husband, the land began to wake. It is at this point that Harry enters her story -- and she confirms that he must be dead, for she can’t sense him anywhere.

November: The velvet-clad Doctor, Harry and Godric return to the woods only to find that the growth is impenetrable. While trying to push his way through, Harry encounters a dryad who attempts to seduce him into her tree, but the Doctor forces her to release his friend. The Doctor explains that dryads have the power to affect men’s memories, thus ensuring that they remain hidden from the world, and when Godric sees the dryad he remembers that she also tried to seduce him. He resisted her wiles, but remained trapped in her tree for 1,000 years while the land slept, only to be released when it awoke again.

The Doctor, Harry and Godric return to the Leffy house to offer their condolences to George, but find that he isn’t at all fazed by his fiancée’s brutal murder. Confused and unsettled by the new world around him, Godric makes an excuse and leaves early, and shortly afterwards, Emmeline arrives, claiming to have recovered from a dizzy spell. The Doctor takes Godric back to his cottage, arranging to meet Harry again later that night, and Harry finds himself in the uncomfortable position of fending off Emmeline, who seems to have taken a romantic interest in him and is unabashedly direct about her intentions. Harry retreats to his room and waits for the rest of the household to fall asleep, but as he sets off to join the Doctor and Godric, he trips over the rug outside Emmeline’s room, attracting her attention. Emmeline invites Harry into her room “to make sure it’s safe,” but the mortally embarrassed Harry stumbles away from her and opens the room’s curtains -- and, when hit by the moonlight, Emmeline transforms into a wolf and flees from the house.

Harry tells his story to the Doctor and Godric, but, while shaken by what he’s seen, he admits that Emmeline seemed to ask him for help as she changed and claimed that she didn’t commit the murder. The Doctor leaves Godric on guard at the house and takes Harry to the graveyard to exhume Lucinda Ryan’s body, and a brief examination of her wounds confirms that the werewolf attack was faked. Someone stabbed Lucinda repeatedly and then used a fake claw to make it appear that she’d been mauled by a wolf.

December: Sarah visits St Sebastian’s and speaks with the mad George Stanton, who claims that he is the true King of England -- and that “the lieutenant” tried to interfere with his mother’s final blood offering. But George will not say whether the lieutenant lived or died. Frustrated, Sarah returns to the village to wait for the Doctor, but dreams of Harry rotting in the ground and wakes determined to know whether the grave really does contain his body. She thus sets off to dig up Harry’s grave, but the labour leaves her exhausted and her hands chafed and bloody. When she opens the coffin she finds that it in fact contains a set of books, but when she rests her hand against the side of the pit, her blood soaks into the earth -- and the pit collapses, casting her into the coffin and burying her alive.

The Fourth Doctor returns to the village with Emmeline, who senses that the land remains uneasy after the events of a fortnight past. She and the Doctor hear Sarah scream, but by the time they reach the cemetery, Harry’s grave has completely collapsed. She will have suffocated by the time the Doctor can dig her out, and Emmeline thus offers her help; if the Doctor can force the change as he claimed, she will let him do so, and as a wolf she will be able to dig Sarah out of the grave more quickly. The Doctor thus fetches a moon rock from the TARDIS, dissolves it into a liquid, and gives it to Emmeline to drink; with part of the moon inside her body, she will be forced to change into wolf form. The plan works, but when the werewolf Emmeline digs Sarah out of the grave, the scent of blood overwhelms her, and she bites Sarah and then flees into the forest. The Doctor revives Sarah, and they set off in search of Emmeline; the Doctor must put right what he’s done by changing her back, while Sarah must find out whether the bite will turn her into a werewolf as well.

November: The velvet-clad Doctor and Harry return to the manor, where they find Godric unconscious, as if struck down by a magic spell. The werewolf returns but is unable to get past them for some reason, and the Doctor and Harry carry Godric inside the house, allowing Emmeline to get past them and change back into human form. When the Doctor and his friends enter her room to confront her she collapses in agony, and after some experimentation, they determine that the source of her pain is Godric’s rucksack. Inside the rucksack, the Doctor finds a remarkable goblet which Godric claims to have found in a freshwater spring shortly before the dryad seduced him into her tree. The Doctor determines that the goblet is nearly 2,000 years old; it must be an object of great power to have caused Emmeline such pain; and, according to legend, water sprang up where Joseph of Arimathea buried the Holy Grail...

Before the Doctor can theorise further, the manor’s plants suddenly lash out, as another blood sacrifice is performed nearby and the land stirs in its sleep. When the Doctor and his friends try to gather the rest of the household together, however, they find that they’re the only ones in the house. Soon afterwards, Trelawney the butler arrives with Lady Hester, who appears to have been wounded when the trees stirred to life again. She claims that she was looking for George, who had joined the hunt for the werewolf. She also claims that the wolf has killed again, this time a local farm girl named Rose Perry. However, Emmeline was in the manor with the Doctor, Harry and Godric know Emmeline was with them when the land stirred -- and when she smells the blood on Lady Hester’s clothes, she realises that Lady Hester is the killer. Hester tries to cast a spell to stun her enemies, but her magic doesn’t affect the Doctor, and she flees. Emmeline pursues her but loses her outside, and when the Doctor and Harry follow, they find the body of Jane the maid, who appears to have fallen and struck her head. They theorise that Hester intended to use Jane as a sacrifice, but was forced to look elsewhere when Jane was accidentally killed while fleeing from her.

George returns to the manor, but is unfazed by the murders and happily confides in Harry that his mother has told him everything; it seems he is the reincarnation of Arthur’s son Mordred, and his mother is the reincarnation of Morgan le Fay. Convinced that Edward VIII will soon be forced to abdicate, Hester has been arranging for her son to take his place. Harry reports to the Doctor, who concludes that whether this is the case or not, Lady Hester presumably believes it -- or is taking advantage of her easily led son. In any case, George spoke of “big plans” for tonight, when the full moon will be at its peak and Emmeline will have no control over the wolf side of her nature. The Doctor concludes that Lady Hester somehow arranged for Emmeline to visit her at this time, and that if Hester completes the sacrificial spells she’s casting, she will wake the land and bind it to her will -- and rule England from behind the scenes, with her foolish son on the throne.

December: While searching for Emmeline, Sarah meets the dryad, who recognises her and confirms that Sarah and the Fourth Doctor arrived in the forest a fortnight ago and took away two men, one of whom matches the description of Harry Sullivan. The dryad then offers to take Sarah into her tree briefly and determine whether she’s tainted by the werewolf curse. She assures Sarah that she prefers the company of men and will release her once she knows whether Sarah is free of the curse. She lies, but Sarah’s garrulousness irritates the dryad so much that she expels Sarah from the tree in any case. She is thus reunited with the Fourth Doctor, and confirms that Harry is still alive and that, once they’ve dealt with Emmeline, they can -- and will -- go back in time and rescue him.

November: The velvet-clad Doctor forms a plan to defeat Lady Hester. Godric has heard of a crystal cave on the other side of the woods, rumoured to be Lady Morgan’s seat of power. Since Godric can hold the Grail without harm but it reacts violently to unnatural elements, the Doctor believes that Godric can use it to prevent Hester from twisting nature to her will. He also believes that the dryad will help them to cross the tangled forest and stop Hester. Harry and Godric have already forgotten meeting the dryad, and the Doctor, who is unaffected by her spell, writes an explanation in ink on their hands so he won’t have to keep explaining himself. Godric fears that he will prove a coward and a failure when the time comes, but Harry assures him that all will be well.

The Doctor decides to get Emmeline out of the village to a place of safety, but they find the road blocked with vegetation -- including wolfsbane, planted by Lady Hester to keep them where she wants them. Unable to penetrate the thicket, the Doctor decides to leave Emmeline tied up in his cottage, and asks Harry to watch over her while he and Godric confront Hester with the Grail. While Harry guards Emmeline, she attempts to flirt with him, sensing that he is a good man and desperate for a husband so she will be able to remain in England. The wolf in her then senses the approach of moonrise, and as she begins to struggle with her bonds, George breaks into the cottage and coshes Harry with a sock full of sand.

Before Emmeline can free herself, George clips a silver collar around her neck and drags her to the village square as his mother has ordered. He reveals that Emmeline is not his cousin after all; Hester used her magic to find a werewolf, and planted false memories in Emmeline’s mind to draw her here. As the full moon rises, George calls the villagers to the square and releases Emmeline, claiming to have captured the werewolf. Emmeline is unable to control herself, and if she slaughters the angry villagers who are attacking her, the blood spilled will be so great and powerful that the land will wake fully from its slumber.

Harry recovers from the blow struck by George, and as he stumbles out of the Doctor’s cottage he hears the commotion in the village square. He tries to stop the villagers from attacking Emmeline, but the wolf recognises him as her chosen mate and attacks, trying to turn him into a werewolf. Fortunately, he is holding a silver candlestick and she is forced to retreat. The Doctor and Godric also hear the commotion, and Godric chooses to return, feeling that he will not be fit to bear the Grail if he turns his back on those in need. When they arrive at the town square, the Grail drives the werewolf away -- but one of the farmers tries to shoot it as it flees, and inadvertently hits Godric. When the shaken farmer tries to help the young man, he is driven back by the Grail, which he finds he is unable to touch.

The Doctor and Harry take the injured Godric to the Doctor’s cottage, where the Doctor applies a poultice and ensures that Godric will survive; however, he is not fit to confront Hester, and the Doctor decides that he must do so himself. But when the Doctor touches the grail, a blast of power knocks him senseless. Harry, who had thought this Doctor to be a good man, is shaken by what the Grail has done to him; nevertheless, when Harry touches the Grail himself, nothing happens, and he realises that it’s up to him to use the Grail against Hester.

Harry carries the Grail to the forest, but can’t get through the thick growth. However, while searching for a way through, he sees the writing on his hand, recalls the existence of the dryad, and calls on her for help. She responds, but his attempts to reason with her fail; nature gives nothing without expecting something in return, and she will only help Harry if he agrees to give her his company. Since the Fourth Doctor and Sarah still haven’t returned for him, he reasons that they must have answered the Brigadier’s call and found Harry alive and well in their own time. He thus agrees to join the dryad in her tree for the next forty years, unaware that if Hester is defeated the land will go to sleep and he will be trapped in the tree indefinitely, as was Godric. In the village, the mob breaks up, shamed by what they’ve done to Godric -- but the wolf stalks and kills the individuals who harmed her and her friends, while other villagers are killed by the land and plants stirred to life by Hester’s magic.

December: The Fourth Doctor and Sarah have pieced together most of the story from what they were told by Emmeline and the dryad, and they conclude that Harry defeated Hester but that the land is still unable to rest after being woken so violently. To wake the land required magic so powerful it was part of Lady Hester’s very being, and to put the land to sleep again will require the same level of magic. The Doctor, however, has another plan. He convinces the innkeeper to give him a pair of raw lamb chops, uses them to lure Emmeline to him and then, to Sarah’s horror, stabs the wolf in the heart with a steak knife. The werewolf’s blood spilled on the ground is powerful enough for the Doctor to send the land back to sleep without further magic, and Emmeline then recovers, as the knife was stainless steel, not silver.

The Doctor fetches Emmeline’s clothing from the Leffy house, and, at her request, returns her to Germany; she has accepted that she cannot find refuge in England, and therefore chooses to try to free her fellow German werewolves, though the Doctor isn’t optimistic about her chances. Before she leaves, she assures Sarah that, in order for a werewolf’s bite to change its victim, the werewolf must want the victim to change.

November: Thanks to his deal with the dryad, Harry is able to make his way through the forest to Morgan le Fay’s quartz cavern. George is sitting on a crystal throne, awaiting his “coronation,” but Harry distracts him by convincing him that the coronation must be accompanied by hymns. As George sings, Harry slips past to confront Hester, who is casting her spells to wake the earth. When Harry lifts up the Grail, the earth convulses, opening up and swallowing Lady Hester. Harry drops the Grail, which is also swallowed by the earth. As George wails and tries to dig his mother out of the earth, Harry stumbles away to keep his appointment with the dryad -- but the werewolf finds him and bites him first, acting on instinct and recalling that she wanted Harry as her mate. Harry gets past the wolf and enters the dryad’s tree, but the dryad senses his impurity and rejects him. He is expelled from the tree just as the TARDIS materialises, bringing the Fourth Doctor and Sarah back in time to rescue Harry just as the dryad told them they had.

Godric recovers in the velvet-clad Doctor’s cottage to find that Harry has gone and the Doctor is lying in a coma. Feeling that he has failed in his mission, the despairing Godric returns to the forest to beg the dryad to take him back, but Harry spots him just in time and convinces the Fourth Doctor to take a side trip and return Godric to his home. Still shaken by what the Grail did to the other Doctor, Harry chooses simply to take his leave and try to remember the other man as a good friend. The TARDIS dematerialises, and the Fourth Doctor never learns that it was originally drawn off course by the electronic equipment connected to the mysterious blue box in the other Doctor’s cottage.

When dawn breaks, Emmeline, back in human form, returns to the velvet-clad Doctor’s cottage and wakes him. She can no longer sense Harry’s presence, and therefore believes him to be dead. Worse, she now knows that she can’t trust her memories, and realises that she can’t be sure whether the horrors she experienced in Germany really happened. The Doctor sympathises, more than she can know. He and Emmeline find their way to the cave, where the raving George Stanton tells them of his mother’s death. They misinterpret his ravings and conclude that Harry and Godric were killed along with her. The Doctor decides to bury coffins full of books in the cemetery so nobody will start asking awkward questions and possibly end up unearthing the Grail again. Emmeline tells Trelawney that Lady Hester, Harry and Godric are dead, and the Doctor arranges for George to be committed to St Sebastian’s -- and offers to introduce Emmeline to his friends in the Ministry, whom he believes will help her to remain in Britain.