An alien survey team is in trouble on a primitive planet. As the weather continues to deteriorate, their fleet leader orders them to return and activates an automatic recall procedure, forcing the ship to initiate its launch sequence. As the pilot Scryfan waits anxiously for his brother Sancreda to return, the pack of wild animals which has been circling Sancreda all day finally works up the courage to attack. Sancreda, caught off guard, loses his focussing amplifier while driving off the pack, and is thus unable to teleport himself back to the ship. Sensing his weakness, the animals close in again, and he fires wildly into the blizzard as he rushes back to the ship on foot. However, he is too late, and the last thing he hears as the ship takes off without him is the voice of his brother, bidding him farewell...

Some external influence draws the TARDIS off course, causing it to materialise in Cornwall. As the Doctor and Evelyn explore, they meet the cheery Mrs Moynihan, who is out walking her dogs Buster and Ben, and who directs them towards the nearby village of Pengriffen. The Doctor is more interested by her mention of a nearby fogou, and Evelyn indulges his curiosity and accompanies him to the fogou, a Celtic underground passage unique to Cornwall. This particular fogou is located near a Neolithic tumlus, a Stone Age burial chamber which predates it by at least two thousand years. As they explore, Evelyn finds and keeps a small, oddly-shaped pebble as a souvenir, and she and the Doctor notice a picture of a demonic figure in the lintel, possibly a protective deity. They are called out of the fogou by young Philip Ludgate, an archaeological student who warns them that the area is dangerous -- a claim which the Doctor dismisses until he notices that Ludgate’s portable equipment is picking up anomalously high electrical emissions from the landscape. Ludgate, intrigued by the Doctor’s knowledgeable questions, invites him and Evelyn back to the Lanyon Moor Archaeological Institute -- a name which Evelyn finds strangely familiar.

The Institute itself is a small set-up in Sir Archibald Flint’s gameskeeper’s lodge, where Professor Morgan is sorting and classifying the relics he and Ludgate have unearthed. Although sceptical of Ludgate’s claims to have detected bizarre electrical activity on the moor, he has allowed Ludgate to carry out his investigation while he gets on with the real work -- assisted by the retired Brigadier Lethbridge-Stewart. Mrs Moynihan returns from her walk and puts on a pot of coffee, and Sir Archibald himself pops in to see how the dig he’s funded is getting on. He’s also interested in knowing whether Ludgate has found the so-called “Spectre of Lanyon Moor”, a joke name for the electrical disturbances he claims to have detected. Ludgate arrives with the Doctor and Evelyn, and when the Doctor warmly greets the Brigadier, his old friend doesn’t have a great deal of trouble deducing who this eccentric man with the strange clothing must be.

As Flint makes his apologies and leaves, the Doctor studies the relics and finds something rather disturbing which warrants further investigation. Morgan, however, is put off by the Doctor’s overbearing attitude, and flatly refuses to let him examine the relics more closely -- particularly when the Doctor claims that one of the relics, supposedly a bronze axe head, is in fact an alien alloy cracked by exposure to the intense cold of deep space. The Brigadier politely but firmly requests that the Doctor be allowed to examine the relics, and threatens to make his presence here official if Morgan becomes obstructive. Furious, Morgan storms off to the terrace with his coffee, while Mrs Moynihan makes her farewells; she’s off to Greece for her holidays and the others will have to make coffee for themselves now. As she goes, Ludgate privately tells the others that her forced jollity masks a deep bitterness; her husband left her for his secretary, and she fell out of touch with her daughter when she married someone of whom Mrs Moynihan disapproved.

The Brigadier informs the Doctor and Evelyn that only Morgan and Ludgate know that he’s not really here on holiday, as the others believe. Back in 1940, a radar installation on Lanyon Moor had to be shut down due to equipment failure and a high rate of nervous breakdowns amongst the staff, and a couple of weeks ago one of Morgan’s assistants had a breakdown in the fogou and has been confined to hospital ever since. The Brigadier was thus asked to visit the site unofficially, to keep an eye on things and alert UNIT if there is indeed something wrong here. The Doctor, intrigued, asks Evelyn to research the history of the area, and at Ludgate’s suggestion she decides to visit Sir Archibald’s extensive library. As she takes her leave of the others, the Doctor studies the remaining relics from the cremation cyst -- mainly the remains of jewellery and a few bones which survived the flames. He finds something which disturbs him greatly, and takes the Brigadier up to the fogou to investigate further. Cornish folklore is full of tales of imps and goblins, and the Doctor and the Brigadier both know very well that sometimes these legends are more than mere tourist attractions. Using a Shlangiian power cell and a roll of copper wire, the Doctor begins to hunt for evidence that the electrical disturbances Ludgate detected are somehow being focussed through the fogou.

Flint appears only too happy to grant Evelyn access to his home, which is rather empty while his butler is off on holiday. After showing her to the library he heads off to take an afternoon nap, apologising for his age, but when Evelyn follows him moments later to ask where the history section is she finds no sign of him anywhere. Left to her own devices, she soon finds what she’s looking for, and makes a quick phone call to her old friend Gareth which confirms her nagging feeling that she’d heard of Lanyon Moor before. As evening approaches, Flint returns and shows Evelyn out; she will make her way back to the Institute on foot, as she needs some fresh air after spending the day in a dusty library. But as she departs, Flint’s other visitor emerges from the shadows, and the two of them retire to the laboratory to prepare for the night’s experiment. Evelyn is too inquisitive for her own good, and Flint feels that something will have to be done about that.

The imp tears Nikki apart but vanishes into thin air when it tries to attack Evelyn. Ludgate arrives, looking for her, and helps the badly shaken Evelyn back to the Institute. It isn’t until the next morning that she feels calm enough to discuss her experience with the Doctor and the Brigadier, who have confirmed that some of the stones in the fogou are of alien origin. When the Doctor learns that the creature was unable to attack her, he theorises that she was protected by some piece of the alien’s technology, and confirms that the odd pebble she picked up in the fogou the day before is in fact a psionic induction loop. Evelyn goes over her discoveries of the previous day; as she’d thought, and a friend of hers who lectures at Oxford confirmed, Lanyon Moor has long held a reputation for ghostly apparitions. During the English Civil War, a party of Roundheads was apparently torn apart by wild beasts while camped around tumulus. When Percival Flint, one of Sir Archibald’s ancestors, tried to excavate the tombs, a diabolic, shrieking wind drove his workers away. And in the 1840s, when a tenant farmer tried to flatten the tumulus, he dropped dead of a heart attack, his entire herd of cattle died within a month, and all crops in the area failed for the next seven years.

The Doctor loses his temper when Morgan dismisses the stories as superstitious nonsense, lashes out at him for making a virtue of a closed mind and mocking anyone who dares to live in a larger Universe than him, and forbids him from approaching the fogou until he has traced the source of the alien psionic energy. As the Brigadier tactfully ushers the apopleptic Morgan from the room, the Doctor advises Evelyn to get some sleep, and she agrees on condition that he apologise to Morgan; the man is simply unable to cope with the supernatural events that have disturbed what should have been a straightforward archaeological dig. The Doctor leaves her to rest, descends to the ground floor and apologises to Morgan for his outburst. Morgan, already calmer, is caught off-guard by the Doctor’s apology, and although still miffed he agrees to leave the fogou alone for the moment. The Doctor, satisfied, returns to the TARDIS to collect his equipment... only to find that the TARDIS is no longer there.

Ludgate fetches some hot chocolate for Evelyn, who has been thinking about Sir Archibald’s disappearance the day before and has come to the conclusion that there’s something odd about all of his servants having gone on holiday at the same time. She has no real proof that anything is wrong, but when Ludgate jokingly suggests breaking into the manor to find out more, Evelyn decides to do just that. Somewhat taken aback, Ludgate accompanies her to the manor, and although they are both having second thoughts by the time they arrive, Evelyn isn’t willing to turn back now. She and Ludgate split up to search for evidence that Sir Archibald is not all he seems, and just as Evelyn is about to give up she stumbles across a secret entrance to a laboratory filled with electronic equipment. Sir Archibald catches her there, and she claims to be alone, which is unfortunate -- for Sir Archibald does not intend to let her tell anyone what she has found...

The Doctor returns to the Institute, aware that time is running out; the creature is already trying to stop him from interfering with its plans, and he doesn’t even know what those plans are. Using bits and pieces from his pockets and Ludgate’s monitoring equipment, the Doctor improvises a tracking unit which will enable him to locate the source of the alien energy. Unfortunately, the creature, although dormant, is still aware of its surroundings and will no doubt try to generate a jamming signal the moment the Doctor switches on. He had intended to circumvent this problem with the TARDIS telepathic circuits, but now he’ll have to use his own mind -- which could prove fatal if the creature is as strong as he fears. But at least one person has died already, and the Doctor therefore prepares to connect himself to the tracking unit, warning the reluctant Brigadier not to switch off the machine until he’s finished, no matter how much pain he appears to be in.

Before Flint can carry through his threat, the Doctor switches on his own device back at the Institute, and the feedback causes Flint’s machine to overload and burn itself out. Furious, he locks Evelyn in his study, telling her that this is only a temporary respite; soon he and his assistant will repair their equipment and the “experiment” can resume. Evelyn, despite her fear, pulls herself together and starts looking for a way out. There are no weak spots in the walls, which just leaves the window; and, left with no choice, Evelyn wraps her cardigan around a book, smashes the priceless stained glass window, and climbs out.

The psychic feedback is far greater than the Doctor had expected, and he is left thrashing about in agony as a telekinetic storm tears the Institute apart. The Brigadier is forced to disobey his orders and switch off, and Morgan arrives to find the Doctor queasily stirring back to life amidst the ruins of his survey. A cup of tea later, the Doctor is feeling much better physically, even if he’s far more worried about the creature than ever before; if it’s this strong while dormant, what will happen when it wakes? Even Morgan is starting to believe that there is an alien entity hibernating beneath the tumlus, and that its powers are roused whenever its rest is disturbed. The Doctor now believes it to be a Tregannon, one of a race of vicious aliens who can regulate their mental activity through the use of psychoactive minerals found in their part of the galaxy. But he’s got some even worse news; before the Brigadier switched off, the Doctor detected a secondary psychic field somewhere near Athens, Greece -- which is where Mrs Moynihan went for her “holiday”. The Celts often used to trade with the Mediterranean civilisations, and it appears that the Tregannon’s focussing amplifier -- which would appear to be nothing more than an ordinary lump of metal -- somehow ended up in Greece... and that Mrs Moynihan has made a pact with the creature to recover it. If the dormant Tregannon gets the focussing amplifier back, it will be able to manifest itself physically on the moor and summon more of its kind to Earth. The Doctor and the Brigadier therefore leave Morgan to bring Evelyn up to date while they set off for the nearby UNIT tracking station, hoping to stop Mrs Moynihan before she returns...

Mrs Moynihan arrives at the Athens Central Museum only to find that the Celtic exhibit has been closed for renovations. Nevertheless, she manages to bribe a tour guide, Pelagia Stamatis, to let her inside for a few minutes. Guided by the voice of Sancreda in her head, Mrs Moynihan soon locates the focussing amplifier, but Pelagia becomes unnerved by her odd behaviour and tries to usher her out. Mrs Moynihan snaps; she’s had enough of having to deal with greedy little people like Pelagia who think they can push her around, and without a second thought, she smashes Pelagia over the head with another exhibit and makes off with the focussing amplifier.

Since the Brigadier retired it has been far more difficult for him to pick up a telephone and mobilise UNIT into action, and it takes far too long for them to act on his report. By the time Captain Ashforde, UNIT’s international liaison, receives word from Athens about the robbery at the museum, five hours have passed; and although the major airports such as Gatwick and Heathrow are being watched, there are still numerous small airports throughout the country which accept international flights. UNIT has placed roadblocks and sentries around the moor, but the Doctor fears that they may already be too late...

Ludgate returns to the Institute to find that Evelyn is still missing, and Morgan leaves him in charge while he goes to the pub to see if the Brigadier has left a message. It is thus Ludgate whom Evelyn finds when she arrives, out of breath from her run. She tells the astounded Ludgate what has happened to her, and decides reluctantly that since there is no sign of the Doctor or the Brigadier, she has no choice but to return to the manor with Ludgate and try to sabotage Flint’s equipment. Ludgate tries to talk her out of it, but eventually, reluctantly, accompanies her back to the manor... where he pulls a gun on her and turns her over to Flint. He has been working with Flint all along, greedy for power and glad for a chance to prove his supposedly crackpot theories. Flint locks her in the wine cellar; he is grudgingly impressed that Evelyn had it within her to smash a priceless Tudor stained glass window and trample a fifteen-foot rose border in order to escape, but he won’t make the same mistake again.

As Sancreda and Mrs Moynihan exult over their victory, Sancreda notices that a vital component is missing from the focussing amplifier. When he describes this induction loop, Mrs Moynihan recognises it as the odd stone which Evelyn picked up from the fogou, and sets off for the Institute to collect it. Meanwhile, the earth quakes beneath the UNIT tracking station as the energy beneath the moor is released, and the Doctor and the Brigadier realise that they’re too late. The Doctor must warn Evelyn of the danger -- but the thought of her reminds him of the induction loop she’d found, and he realises that they may be able to use it as a bargaining tool. They will have to act quickly, however, for the Tregannon ship has already been detected entering Earth’s solar system. Soon after they depart, UNIT’s surveillance satellites start to go down, and within seconds a blackout area has been established. UNIT has been rendered blind over Lanyon Moor.

The Doctor and the Brigadier arrive back at the Institute to find a note from Morgan, who has ignored the Doctor’s warnings and gone out onto the moors to look for the missing Evelyn and Ludgate. Morgan then returns, claiming that the tumulus is split open as if something has burst out from beneath. He volunteers to drive over to the manor to warn Flint of the danger while the Doctor confronts Mrs Moynihan; the Brigadier remains at the Institute in case Evelyn or Ludgate tries to make contact. The Doctor finds Mrs Moynihan waiting at her cottage with her dogs, Buster and Ben, and she drops her charade of cheeriness, realising that he knows the truth. All her life she’s been used and deserted by those she loved; only Buster and Ben have stayed loyal to her. She’s bitter, she’s looking forward to her revenge, and she’s got a gun, so the Doctor is forced to accompany her and her dogs back to the Institute, to wait for Sancreda’s return.

As Flint and Ludgate try to repair the damaged equipment, Ludgate admits he doesn’t want to get Evelyn involved and asks Flint to release her; however, he refuses to do so. Ludgate then hears a cackling sound outside, and is killed by Sancreda when he investigates; Sancreda has come to take revenge on those who would harness his powers and drive him like a pack animal. Flint claims to seek a partnership, and when Sancreda contemputously dismisses his petty dreams of power over the Earth, Flint threatens to activate the machines and turn Sancreda into his slave. But with a single thought, Sancreda pulverises the laboratory, and then turns the full force of his rage upon the terrified Flint. Some time later, Evelyn hears the shaken Morgan calling from upstairs, and he finds and releases her, inquiring after the stone which the Doctor had told him about. She tells him that it’s still in her handbag at the Institute, and she and Morgan depart from the manor, trying not to look too closely at the remains of Flint and Ludgate as they go.

Morgan and Evelyn return to the Institute, where Mrs Moynihan is holding the Doctor and the Brigadier at gunpoint and waiting for Sancreda to arrive. The Doctor tries and to convince her that Sancreda will never honour their bargain, and Morgan agrees -- and sheds his human form, revealing himself to be Sancreda. The real Morgan has been lying dead on the moor since shortly after Sancreda escaped; he shouldn’t have ignored the Doctor’s warning... Sancreda sends Evelyn to fetch his induction loop, and then rewards Mrs Moynihan for her faithful service by planting an empathic suggestion in the minds of her dogs, causing her only true friends to leap on her and tear her apart. He then takes his three prisoners with him as witnesses; his brother is going to pay dearly for abandoning him on Earth for eighteen thousand years. But as the ship lands, summoned at last by the imperative recall which Sancreda was unable to send without the focussing amplifier, the Doctor wonders aloud why Scryfan did not return for Sancreda before.

The answer becomes clear when Sancreda enters the ship to find that Scryfan is not there. When the Doctor first arrived, he realised that some of the bone fragments Morgan had unearthed were extra-terrestrial, and that the alien had been shot with an impulse laser. Thousands of years ago, as Sancreda rushed through the blizzard, Scryfan stepped out of the ship to rescue him -- and Sancreda, firing wildly into the storm as the wild animals closed in around him, shot his own brother without realising it. Scryfan’s last words as the ship departed on automatic recall really were his last words. Sancreda, stunned, decides to avenge himself upon the entire planet Earth by blasting it from space with his psionic cannon. He orders the others off his ship; he cannot release a psychic blast in here for fear of damaging the ship, but he can still shoot them with his impulse laser. As the Doctor and Evelyn retreat, however, the Brigadier manages to catch Sancreda off guard and take the blaster from him; however, it is keyed to Sancreda’s psyche, and the Brigadier is unable to focus its energy into a killing blast. Nevertheless, he threatens to open fire at random if Sancreda tries to kill him, and the enraged Sancreda allows him to depart from the ship, knowing that he and the rest of his kind will be dead within minutes in any case.