Early in the morning, Stephen wakes from his recurring nightmare in which he struggles against a river current while his grandmother floats away, calling for help. He intends to tell Miss Decima what he overheard, but on the stairs, he encounters Lowen, who orders him to prepare the breakfast room instead.

Edwin arrives for breakfast with Jolyon, who is visibly hungover. When Inspector Jarvis announces he has new leads from locals at the tavern and requests access to the cellars, Jolyon reacts nervously. Heading toward the Nursery afterward, Stephen overhears Professor Müller and Lettice arguing. He hides and listens as Lettice blackmails Müller, threatening to reveal what she saw—and produce a photograph as proof—unless he complies by the next day. Stephen now has three major pieces of information: the changed will, Jarvis searching the cellars, and Lettice’s blackmail scheme.

Stephen reports everything to Miss Decima and Temperance in the Nursery. Miss Decima analyzes each suspect’s motive: Edwin has long coveted the family fortune; Jolyon is heavily in debt and always wanted the estate; Lettice wants to gain influence over whoever inherits the estate, and she’s capable of poisoning; and Professor Müller may have been named in the will to funnel funds into the Viscount’s Society.

Temperance theorizes that the photograph Lettice is using for blackmail is from Müller’s photograph of the Viscount on Wednesday, and that it may show evidence of the murder machine. Miss Decima agrees they must find the glass plate and announces they will search the crime scene.

Stephen pushes Miss Decima to the first-floor corridor, where she examines the suit of armor’s crossbow and dismisses it as a fake prop meant to suggest ghostly revenge. She notes the bloodstained wadding’s position was inconsistent with the crossbow’s likely firing path.

Inside the study, Miss Decima tests the oxygen tanks for poison gas but finds nothing. She notices a small symbol in the margin of papers on the desk and goes pale, though she dismisses it. After an hour of searching, they find no evidence of a murder machine or tampering, and Miss Decima concludes their theory is wrong.

She retrieves the original floor plans of Tithe Hall from a card catalog—Conrad had confiscated her books and equipment after plagiarizing her scientific papers—and explains the house’s complex network of servants’ passages. She says she will search the plans for secret entrances to the study, and orders Stephen and Temperance to search Lettice’s bedroom for the glass plate while the family is at lunch.

Stephen and Temperance search Lettice’s empty bedroom thoroughly but find no glass plate. In Gilbert’s adjoining room, Stephen discovers disturbing drawings depicting Gilbert violently killing Jolyon, Edwin, and Stephen himself.

Heading back down the corridor, Stephen notices a butler’s mirror that he is certain was not there on Wednesday. In its reflection, he sees a figure spying on them from down the hall. They walk away casually, monitoring the follower in subsequent mirrors as the figure draws closer. They turn a corner and run, hiding in Jolyon’s bedroom—which is filthy and foul-smelling. As footsteps approach, Stephen pulls Temperance under the bed. The door opens, and from the small feet and heeled shoes, Stephen recognizes the intruder as Lowen.

Hidden under the bed, Stephen and Temperance watch as the room’s overwhelming stench repels Lowen, and he retreats without searching further. Stephen concludes Lowen is trying to get him in trouble out of spite and to eliminate a rival.

Temperance asks Stephen why he went to prison. He tells her that he joined a gang to earn money for his sick grandmother’s care, got into a fight that led to a conviction for grievous bodily harm, and his grandmother died while he was imprisoned. Temperance takes his hand, and they share a brief, charged moment of connection—broken when Stephen discovers a large cache of empty wine bottles hidden under Jolyon’s bed. Temperance does an impression of Edwin that is remarkably accurate, and her vocal mimicry gives Stephen an idea.

Stephen proposes that Temperance impersonate Edwin on the telephone to extract information about the will from the clerk, Wilkins. Miss Decima places the call and coaches Temperance with hastily written notes. Nervous but determined, Temperance successfully mimics Edwin’s voice and pressures Wilkins to talk.

Wilkins reveals that when the Viscount changed his will, he was accompanied by an unidentified man whom no one in the office recognized. Before Temperance can learn more, Wilkins hangs up. The trio registers the development: Conrad had help changing his will. Their discussion is interrupted by shouting from the floor below.

Stephen, Temperance, and Miss Decima rush downstairs to find Inspector Jarvis attempting to arrest Mr. Stokes. Jarvis accuses Mr. Stokes of masterminding the murder and Stephen of being his hired accomplice. He claims Mr. Stokes killed Conrad for revenge after his father was dismissed for stealing wine, and that Stephen shot the Viscount from the doorway before resealing the room.

Miss Decima calmly debunks the theory by pointing out that the keyhole and doorframe were sealed from inside the locked room. Jarvis then presents his final evidence: Six bottles of wine are unaccounted for in the house records. Stephen reveals they are hidden under Jolyon’s bed. Jolyon admits to helping himself to the wine, and Jarvis withdraws, chagrined.

Regrouping in the Nursery, Miss Decima crosses Jolyon off the suspect list, concluding he lacks the cunning to be a criminal mastermind. Stephen argues for removing Mr. Stokes as well, but Miss Decima refuses—while Jarvis’s reasoning was flawed, Mr. Stokes’s movements on Wednesday night still rely solely on his own word.

Miss Decima assigns new tasks: she will study the house’s floor plans for secret passages, Temperance must find the photographic plate Lettice is using for blackmail, and Stephen must follow Lowen to discover his role in the events. As Stephen leaves, Miss Decima warns him to be careful—there is a murderer in the house.

After searching unsuccessfully for Lowen all afternoon, Stephen reports to Mr. Stokes’s office for dinner service. Mr. Stokes mentions that Lowen has been demoted to downstairs work for stealing wine on Jolyon’s behalf. Stephen asks Mr. Stokes directly if he committed the murder. Mr. Stokes swears that although he often hated the Viscount—who treated his staff with contempt despite 20 years of faithful service—he did not kill him. Stephen believes him completely.

Leaving the office, Stephen encounters Lowen in the corridor. Lowen whispers a threat: He knows Stephen is the murderer and will prove it.

That evening, Stephen serves dinner with Mr. Stokes. The family eats in tense silence, and Professor Müller is absent. Stephen remains preoccupied with Lowen’s threat as he performs his nightly duties, closing curtains, locking doors and windows, and extinguishing lights. Unseen by him, the murderer watches from the shadows.

An epigraph from the Jarrow Express dated Sunday, May 22, 1910, describes how inhabitants of a Hungarian village panicked at distant flames, believing the comet signaled the world’s end, and held a frenzied feast in the town square.

At dawn on Sunday, Stephen is awakened by a scream from the servants’ floor. Merryn claims the ghost entered her room and stole a silver cross from under her pillow. Mrs. Pearce arrives, dismisses the story, and the episode ends in embarrassment for Stephen when the maids notice his unbuttoned trousers.

Going downstairs to prepare the breakfast room alone—now covering Lowen’s duties as well as his own—Stephen finds Edwin replacing Conrad’s marble bust with one of himself. Edwin announces the family will attend church in the nearby village of Tregarrick, and that he has invited Reverend Wellbeloved to dinner to help dispel rumors of foul play. Jolyon is excused from church on the condition that he remain sober for dinner. As the family departs, Stephen realizes the empty house presents an ideal opportunity for investigation.

Stephen and Temperance find Miss Decima exhausted at her desk, having worked through the night. When she refuses treatment for her swollen feet, they make it a condition of their continued assistance. She relents, and Stephen begins massaging her feet.

Miss Decima delivers disappointing news: The floor plans reveal no secret passages into the study. She insists they refocus on identifying what changed in the room between Wednesday and Thursday.

Temperance produces the glass photographic plate Müller took of the Viscount in his study on Wednesday, having stolen it from Müller’s room that morning. She clarifies that it is a different plate from the one Lettice is using for blackmail and proposes comparing it to Inspector Jarvis’s crime-scene sketch to identify any differences. Miss Decima agrees and announces she will create a cyanotype print using chemicals from the study. She assigns final tasks—Temperance must obtain the sketch from Jarvis, and Stephen must gather the chemicals—but insists he finish the foot massage before he goes.