It is the winter of 1898, and Banquo Manor -- the home of George and Elizabeth Wallace, and the scene of a gruesome murder in the previous century -- is about to host an experiment in thought transference. At the request of scientist Richard Harries, Wallace has dismissed most of the servants for the weekend, apart from the indispensible butler Simpson and the maid Beryl (whose relationship with Wallace may not be entirely maidenly). Harries has also invited witnesses to lend authority to his experiment, but so far only solicitor John Hopkinson has arrived; German pathologist Doctor Friedlander will be unable to attend due to poor weather, and British scientist Gordon Seavers has yet to respond. For personal reasons, Hopkinson does not reveal that Seavers committed suicide some days ago, and that he was the one to find the body. When Hopkinson arrives, Harries demonstrates his theories to him by running a rat through a maze and then using electrical current to stimulate its natural neural activity; this induces resonance between its brain and the brain of a second rat from the same litter, which acquires the first rat’s knowledge of the maze. Tonight, Harries and his sister Catherine will attempt to use the process to share each other’s thoughts...
Compassion the humanoid TARDIS briefly materialises near Banquo Manor, just long enough to be caught in an artron inhibition field set up by an agent of the Time Lords. In one blow, she loses ninety percent of her artron energy, and to maintain her stability she materialises around a human host -- Susan Seymour, Richard Harries’ fiancee, who had gone for a walk in the grounds to calm down after telling Harries that she was breaking off their engagement. Compassion still has enough power to block the signal the Time Lord is attempting to send to Gallifrey, but as long as the field is in place, she cannot dematerialise and the Doctor cannot regenerate -- and if it is not shut down soon, she will merge with Susan entirely, and lose her own identity. Bearing this in mind, the Doctor and Fitz return with her to the manor, where the others assume that the Doctor is in fact Dr Friedlander and prepare to conduct the experiment. Something goes wrong almost at once, however; Harries’ equipment begins to overload, and although Hopkinson manages to remove the connections from Catherine in time, her brother is electrocuted and horrifically burned by the power surge.
While Catherine is sedated and led to her room in shock, Wallace calls the local police station to report the death. Sergeant Baker arrives in response to the call with Inspector Ian Stratford of Scotland Yard, who has arrived in the area to question Hopkinson about Seavers’ supposed suicide; he wishes to know how Seavers became such a prominent scientist when his early work at Oxford was unpromising at best, and what was in a letter which Seavers had received only that morning and which he suspects Hopkinson had stolen before reporting his friend’s death. Wallace offers to put Stratford and Baker up for the night while they investigate Harries’ death, which the Doctor claims was murder; two wires on Harries’ apparatus had been switched about, causing it to overload. While the Doctor checks for footprints in the snow outside the conservatory where the apparatus had been stored, Stratford and Baker question the other witnesses. The interview with Hopkinson does not go well; Stratford believes that the smug solicitor is hiding something of importance, while the terrified Hopkinson, beneath his show of confidence, is sure that Stratford knows he stole the letter and is simply toying with him.
Simpson claims to have seen the Doctor and Fitz leaving the conservatory before they supposedly arrived at the house, and although Baker notes a suspicious flaw in his story, he and Stratford decide to question the Doctor in more detail once he returns. However, he fails to do so by morning, thus becoming the prime suspect. Stratford decides to examine Harries’ body, only to find that it too has vanished during the night. Although he and Baker search the house from attic to cellar, including Simpson’s remarkably spacious and meticulously orgnaised pantry, they fail to find either the body or the Doctor; however, Baker spots the Doctor’s footprints outside, leading away from the house. Fitz, who is becoming concerned by his growing inability to perceive anything of Compassion in Susan’s behaviour, accompanies Baker and Stratford as they follow the Doctor’s footprints away from the Manor. The trail leads through the woods, past a hut containing blasting equipment for the grotto which Wallace intends to build, and finally to a thirty-foot drop... with the Doctor’s body at the bottom, twisted and broken, head and legs buried beneath the snow, clearly dead. None is more depressed by the discovery than Fitz, who knows that the artron inhibition field will have prevented the Doctor from regenerating, and who believes that Compassion is being subsumed within Susan’s personality; soon only he will be left.
As dinner is served, Simpson reports that Harries’ body has turned up again, and that Beryl is watching over it -- but at that moment, the others hear a terrible scream, and find that Beryl has been strangled to death. In her hand, Stratford finds a scrap of paper containing some of Harries’ notes, which confirm suspicions that Stratford had formed from watching Wallace and Beryl together. When he shows the scrap of paper to Susan, she admits that she broke off her engagement to Harries when she learned that he was blackmailing their supposed friends in order to finance his experiments. Hopkinson admits that Seavers had risen to his esteemed position by taking the work of a dead friend from Oxford and publishing it as his own; Hopkinson stole and burned the letter from Harries which would have exposed him. Stratford deduces that Seavers was driven to suicide by Harries’ threat of blackmail, and that Hopkinson murdered Harries to avenge his dear friend. Since Catherine was the only person absent from the dinner table when Beryl was killed, it seems that she was driven mad by her brother’s death, and is killing anyone who knew of his schemes.
Stratford sends Wallace to fetch his wife, who is resting in her room following the shocks of the past 24 hours, but just as he is about to place Catherine under arrest there is another terrible scream -- and Stratford, Fitz and Hopkinson find that Wallace has been strangled, and Elizabeth smashed into the wall with such force that the back of her skull has been caved in. As there is only one stairwell in the house, the killer must still be upstairs, and while Simpson fetches Wallace’s shotgun, Stratford, Fitz and Hopkinson search the upper rooms for the killer. Instead, they find the Doctor, who faked his own death by stuffing his coat with sticks and throwing it over the cliff, in order to lure the Time Lord agent into a false sense of security while he searched for the artron inhibitor. But he had assumed that there was only one mystery to solve -- and he is proven wrong when Richard Harries’ dead body lurches out of the shadows and attacks them.
The Doctor, Fitz, Hopkinson and Stratford flee downstairs, where Simpson is so surpriesd to see the dead Harries in pursuit that he utters a Gallifreyan oath. He attempts to shoot Harries, but the shotgun does not stop the already dead man, and Harries knocks Simpson over the banister, breaking his leg. In the rush to save Simpson and avoid Harries, the survivors find themselves trapped on the stairwell, and they are forced to flee upstairs and barricade themselves inside Hopkinson’s room. There, even despite his own grievous injury, Simpson refuses to deactivate the artron inhibitor unless the Doctor agrees to surrender Compassion to the Time Lords. The Doctor realises that the inhibitor is drawing power from the natural artron fields of Banquo Manor, which were stirred up by the terrible murder of the previous century -- and that the artron field flux caused by the inhibitor must have interacted with the power surge from Harries’ own equipment, creating a mental link between himself and his sister at the moment of his death. Catherine’s id has rebelled at the thought of her brother as a blackmailer, and she has subconsciously sent his body on its murderous rampage to eliminate all those who knew the truth.
As Harries begins to break down the door, Simpson suggests knocking Catherine out to stop him, but she reaches deep within herself and halts her dead brother in his tracks. The Doctor, Hopkinson, Stratford and Baker then set off to search for chains or ropes with which to confine the undead Harries. But while Fitz and Susan are tending to the injured Simpson, Catherine slips out of the room, seizes Wallace’s revolver and takes her brother to the drawing room to dispose of their enemies; she was aware of Richard’s activities and her control of his body has been conscious all along. Before they can kill the Doctor and his allies, Susan arrives looking for Catherine, and the others take advantage of the distraction to lock Catherine and Harries out of the drawing room. Susan flees back upstairs, and she, Fitz and Simpson barricade themselves inside Hopkinson’s room. Catherine stands guard outside the drawing room and sends Harries to break through the French windows. Stratford, believing that she has accompanied Harries outside, opens the door and is shot in the shoulder before he can slam it shut again. The Doctor, Hopkinson and Baker lock the door again and barricade the windows, and tend to Stratford’s injury. Their only hope is for the Doctor and Hopkinson to climb through the fireplace flue which connects the drawing room to the study, and try to fetch help from the village.
Unable to get into the drawing room, Catherine sends Harries to break into Hopkinson’s room again and kill Fitz, Susan and Simpson. Simpson holds Harries off while Fitz and Susan climb out of the window on a rope of bedsheets, but Harries overpowers the weakened Simpson, gouges out his eyes and throws him out of the window. He unties the bedsheets as Susan climbs down, and she lands badly and sprains her ankle -- disturbing proof for the Doctor and Fitz that in this form, Compassion can be injured and perhaps even killed. Simpson apparently dies, still refusing to tell the Doctor where he has hidden the artron inhibitor, and the Doctor, frustrated, sends Fitz and Susan back through the study to join the others while he and Hopkinson try to draw Harries off through the woods. Harries does not slow down and does not tire, and the Doctor realises that he will catch them long before they reach the village. The Doctor therefore takes a detour to the construction hut, where he and Hopkinson find the dynamite which was to be used to blast out Wallace’s grotto; unfortunately, they have no way of igniting it, and must flee back to the Manor with Harries in pursuit.
Back at the Manor, the generator finally runs out of coal and dies, and the besieged company are forced to light oil lamps for illumination. The Doctor and Hopkinson return, and when Stratford and the others pull aside the barricade to let them in, Harries arrives and smashes his way through before they can replace it. Realising that victory is at hand, Catherine shoots out the lock and enters to watch her enemies’ deaths -- starting with Susan, who nearly took Catherine’s beloved brother from her. But there is still an element of individuality left in Harries’ mind, and he hesitates when ordered to kill the woman he loved. The Doctor and Baker take advantage of his momentary distraction to attack him, and Baker manages to stuff the dynamite into Harries’ jacket before Catherine shoots him in the head. Hopkinson then throws an oil lamp at Harries’ body, which bursts into flame -- and explodes, blowing Harries to bits. The enraged Catherine is about to shoot them all, but Harries’ severed right arm clutches at her in a spasm, as if still trying to protect Susan. While Catherine is distracted, the Doctor and Hopkinson grapple with her, and as they try to get the gun away from her, it goes off, killing Catherine.
The Doctor still has to find the artron inhibitor, and although a search of Simpson’s pantry proves fruitless at first, he eventually realises that it would have to be disguised as an electrical circuit of some kind. When he examines Harries’ ruined equipment, he realises that the sole surviving valve is the artron inhibitor, and as soon as he smashes it, Compassion returns to normal. The Doctor and Fitz depart with Compassion, and as soon as she detaches from Susan, Susan also returns to normal -- but remembers everything that happened to her. And although it takes him seventy years to recover from his terrible injuries and track her down, Simpson eventually locates the elderly Susan on her deathbed and questions her. She still remembers the jolt she felt when Compassion left her -- and she still remembers the numbers which the Doctor fed into the console before Compassion dematerialised. This is the seed code for Compassion’s Randomiser, and with this information the Time Lords will be able to predict with near certainty Compassion’s destination after leaving Banquo Manor. Simpson transmits the information to Gallifrey, knowing that when Compassion next materialises, the Time Lords will be waiting for her...