Night is falling in Excelis, and the Curator of the Imperial Museum is shown out for the night by his associate Solomon. As soon as he’s gone, a thief steps out of the shadows, and Solomon switches off the alarms. Deep within the eerie, haunted museum is the secure storage chamber, where the Restoration Team keeps the projects which are not to be displayed for the public; the Imperial Family has decreed it too dangerous to release the truth about the mysterious Relic. Solomon and the thief intend to steal this Relic, which looks for all the world like a tattered old handbag, but though Solomon assures the thief that it’s perfectly safe, an alarm starts to sound the moment the thief touches the Relic. Steel shutters lower themselves into position around the reliquary, and although Solomon attempts to escape, he only makes it halfway...

The TARDIS materialises in the museum, and when the Doctor emerges to hear alarms and a muffled cry for help, he assumes that there’s a fire and that someone is trapped. However, before he can find a way to open the door and release the trapped man, the curator returns to the museum with Inquisitor Danby, and the Doctor decides to hide and listen in until he knows what’s going on. Danby and the Curator open up the shutters to find that Solomon has been crushed to death beneath them. The thief makes no attempt to hide his obvious guilt, but claims that Solomon was the brains of the operation. Danby has the thief locked up in a secure storeroom to await questioning, while the curator examines the sanctum and determines that the Relic is the only object out of place. The Doctor has returned to the TARDIS, and is watching and listening to everything on the scanner -- and he’s surprised to learn that he is once more on Excelis, and once more involved in a mystery surrounding the Relic.

The Curator spends the night accounting for the artefacts in the museum, and is satisfied that only the Relic was tampered with; however, he can’t explain the presence of the large blue box which wasn’t there the night before. Danby questions her wardens to find out if the object was delivered while they were there, and the Doctor takes the opportunity to slip out of hiding and introduce himself to the Curator. When the Curator explains that the museum is closed, the Doctor offers his help -- but when Danby meets him, she immediately sees through his claim to have just arrived. The Doctor changes tactics, claiming that he heard the alarms last night and entered under the impression that someone was trapped in a fire -- but if that’s true then he must have locked the door behind him on his way in. Suspicious, Danby takes the Doctor to the storeroom to speak with the thief, who confesses that he was hired to steal the Relic by an anonymous third party. He recognises the Doctor’s voice from the night before, but his story confirms the Doctor’s claim that he was investigating what he believed to be a fire alarm. The Doctor explains that he didn’t introduce himself immediately for fear of ending up in this very predicament, and Danby decides to give him the benefit of the doubt -- but orders him to remain in the museum until her investigation is complete.

Reeve Maupassant, head of the wardens’ bureau and the most important civilian official in the city, arrives to take personal charge of the investigation, claiming that he prefers not to remain deskbound. In fact, he has an entirely different reason for being here -- and a far more colourful past than it seems the Curator can imagine. Maupassant suggests that the Relic be placed in safekeeping at the wardens’ house, but Imperial decree forbids the Curator to hand it over to anyone but the Empress, the Regent, or the Etheric Minister. Maupassant swallows his disappointment, and is then distracted by his sight of the strange blue box -- which he seems to recognise as a “TARDIS”, although he can’t quite remember what that means. It comes to him soon enough, for although he fails to recognise the colourfully-clad stranger who was just speaking to Danby, he immediately recognise the name of “the Doctor”. But this Doctor was nothing like the man Maupassant remembers. Maupassant goes straight to the tea room, where he finds the Doctor and the Curator discussing Excelis’ war with its rival countries. He sends the Curator to prepare the scene of the crime for study, but first speaks to the Doctor about the TARDIS, and sees a spark of recognition in his eyes. He thus orders the Doctor to stay in the museum until the investigation is complete, but the Doctor has no intention of leaving -- not until he learns how the man calling himself Maupassant has lived for over a thousand years.

Maupassant and the Curator study the Relic, and Maupassant reveals that he’s read the records in the Imperial Archives; all the stories about it are true, and Maupassant himself has very good reason to believe that the warlord Grayvorn did indeed open the Relic and disappear along with the Mother Superior of the Little Sisters of Excelis. The ancient handbag appears torn and tattered, but the clasp is fused solid and there’s no way to tell whether the thief’s rough handling has damaged it. Thus, the Curator suggests fetching the Etheric Minister, an old friend, to examine the Relic without opening it. Maupassant is forced to agree to this, but then he suffers a strange fit and locks himself in Solomon’s office, lapsing into delirium and referring to himself as “we”. Voices clamour in his head, especially one strident female voice which speaks to him directly -- and calls him Grayvorn.

Concerned about the Reeve, the Curator calls on Danby and the Doctor to see to him. Maupassant is screaming in agony, but has locked himself inside Solomon’s office. By the time the Doctor breaks down the door, the Reeve has recovered from his fit and dismisses it as unimportant. The Curator and Danby leave him to recover while they prepare for Pryce’s arrival, and much to the Doctor’s surprise, he learns that they intend to conduct a seance to commune with Solomon. In Excelis, the spirits of the dead still have their part to play in law enforcement, but Maupassant knows that shouldn’t come as a surprise -- didn’t the Doctor once see a community of zombies who survived beyond death thanks to the Relic? The Doctor realises that Maupassant is indeed the man who was once known as Grayvorn. Minister Pryce is here now, and he outranks the Reeve, but the Doctor can’t exactly go forth and claim without proof that Maupassant is really a thousand-year-old warlord. However, it would be easy for Maupassant to prove that the Doctor is not a native of Excelis, and have him arrested on charges of espionage...

Pryce warmly welcomes his old friend the Curator, and has the museum sealed in preparation for the seance. The Doctor offers to help prepare the room, despite his scepticism that anything will come of it. Danby remains with Pryce, who mentions having seen ghosts in this building when he worked here as a student; the current thinking is that certain buildings, particularly old stone structures, store intense emotional impressions from traumatic experiences, which can later be replayed to sensitive minds.

Thanks to the Doctor, the room is prepared for the seance ahead of schedule -- and this gives the Doctor time to discuss Maupassant with the Curator. There have been wardens and Reeves in the city for 300 years, but photography was only discovered earlier in this century. At the Doctor’s urging, the Curator takes him to the archives to look at the oldest photographs of the wardens -- and as the Doctor had suspected, one of the street wardens in an old photograph is Maupassant, or rather, Grayvorn. The Curator seems strangely open to the Doctor’s wild claims, and when pressed, he admits that he’s seen this photograph before. He recognised the Reeve the moment he arrived, but simply couldn’t believe his eyes. The Doctor explains that when he knew Maupassant as Grayvorn, he was desperate to control the Relic -- and it’s unlikely to be a coincidence that he’s shown up now that there’s trouble with the Relic once again. When the Curator casually mentions that the Mother Superior disappeared at the same time as Grayvorn, the Doctor realises that the Relic must have merged their minds together; it’s more powerful than he’d imagined. But he still doesn’t know what Maupassant is planning to do, and that question will have to be delayed, as the seance is about to begin.

The Doctor and the Curator check in on the thief, who seems well if uncomfortable, and then attend the seance. Despite the Doctor’s scepticism, he watches as Pryce places himself in a trance and tries to commune with Solomon’s spirit. Solomon speaks from the ether, comfortable and peaceful in this strange new environment, and admits that he conspired to steal the Relic. However, he also claims that the Doctor was his fellow conspirator. Maupassant tries to place the Doctor under arrest, and insists that he be taken to the warden house -- along with the Relic, which is required as evidence. Pryce, however, claims that the link to the spirit world will remain open until dawn, and that everybody who participated in the seance must remain in the museum until then, for fear of taking others out with them. Maupassant reluctantly departs to advise the other wardens of the situation, and Pryce privately admits to the Doctor that he knows something is wrong. They all heard the words which Solomon spoke, but Pryce sensed that someone else was forcing his spirit to speak them...

Maupassant returns and sends the others away while he questions the Doctor privately. He admits that he knows torture won’t extract any useful information, but that doesn’t mean he won’t resort to it to make a point. This established, he demands to know how the Doctor has changed his physical form. The Doctor claims that Maupassant would never understand, so the Reeve confines himself to simple yes or not questions. Is the Doctor’s current body different from the body in which he met the warlord Grayvorn? In “yes or no” terms, yes. Is the mind inside that body the same mind? Yes. In that case, Maupassant knows all he needs to -- or so he believes. However, he still intends to keep the Doctor locked up, in order to keep him out of the way while he reclaims the Relic and seizes control of all the souls in the afterlife.

Irritated, the Doctor demands to know why Maupassant is still so obsessed with the Relic, when it’s cursed him with immortality and merged his mind with that of the Mother Superior. He believes that it must be a living hell, but in fact it’s even worse than he dreamed -- for the Reeve has not slept for one moment in a thousand years. The humanoid brain is not designed to function without dreams, which it needs to process the events of the day -- and so Grayvorn dreams while awake, experiencing waking hallucinations, intermittent periods of madness like the one he suffered earlier. The Doctor is appalled, but Maupassant claims that he’s adapted to his circumstances... and insists that he’s still perfectly sane.

Danby checks in on the thief, only to find that he and the Curator have both been murdered. It seems that the thief had yet another accomplice, who is still at large. Maupassant departs to ensure that the Relic is still under guard, and the Doctor, frustrated, admits to Pryce that he knows who the killer is... but that he can’t make any accusations without proof. However, he’s forgotten that dead eyewitnesses on Artaris can still tell tales -- and the killer has made a grave error in killing two men at the same time, for even if he can control spirits through his link to the Relic, he can’t speak in two different voices simultaneously. The Doctor thus urges Pryce to conduct another seance -- this time, without the Reeve present. Pryce does so and contacts the spirits of the Curator and the thief, who are confused but serene in their new world. They reveal that Maupassant killed them; the Curator because he knew who Maupassant really was, and the thief because he’d worked out that Maupassant hired him. He’s also worked out that his attempt was meant to fail. If the Relic vanished every warden in the city would be searching for it, but if someone tried and failed to steal it, the Reeve could arrange for it to be handed over to him as evidence.

The Doctor reveals that Maupassant is really the legendary Grayvorn. Pryce finds this hard to believe, but nevertheless sets off to arrest him for the murders of the thief and the curator. However, soon after Pryce departs, the Doctor finally works out what Maupassant intends to do, and realises that Pryce is in danger. Maupassant believes that the Doctor has transferred his mind into another body -- and two minds currently occupy Maupassant’s body. Whether he intends to expel the Mother Superior from his immortal body or himself escape into another host, he needs only the Relic and a host for the second soul -- and he’ll have no scruples about killing Pryce to get what he wants. The Doctor and Danby rush to stop him, and find that he’s already knocked out Pryce and is struggling to open the rusted clasp of the Relic. Danby tries to stop him, but he’s sure that no woman can overpower him... until she kicks him in the place which can overpower any man, and wrestles the Relic away from him. The Doctor opens it and threatens to look inside himself, and when Maupassant rushes to stop him, the Doctor turns the open Relic on him. It blazes with power, momentarily revealing the spirit of the Mother Superior, but when the blaze clears, she and Maupassant have vanished, perhaps into the Relic. The Doctor snaps the Relic shut, closing the case in every sense of the term.