Two hours later, Chief Constable Penrose arrives with a police surgeon and officers. They search the study, the house, and the grounds but discover nothing. That afternoon, Penrose meets privately with Edwin and informs him that Inspector Jarvis from Scotland Yard, who knew the Viscount personally, will arrive the following day. Postmortem results will be ready on Monday. Penrose warns Edwin to prevent staff from gossiping in the village. After the police depart with the body, Edwin summons everyone to the drawing room.

Edwin formally announces the Viscount’s death. Stephen observes the family’s reactions: Jolyon appears drunk and disheveled; Lettice fusses over Gilbert; and Professor Müller, the German guest, looks visibly shaken. Edwin declares a complete lockdown—no one may leave or enter the island, write letters, or speak with mainland tradesmen until after the Inspector concludes his investigation, and no guest may depart before the will reading the following Friday. Edwin announces he is assuming control of the estate.

Afterward, Stephen secretly returns to the study, certain that something has been altered since his first visit the previous night. Professor Müller suddenly enters, startling them both. Stephen flees and, using a butler’s mirror in the corridor, sees Müller watching his departure with a guilty expression.

Stephen joins the staff in the servants’ hall where Cook is anxious about the murder. Lowen mentions the death mirrors that of the Viscount’s father, and Mrs. Pearce reveals the previous Viscount died in a hunting accident—shot in the eye with a crossbow. Lowen shares old gossip that the current Viscount was suspected of murdering his father for the inheritance. Merryn, a young maid, claims the father’s ghost has taken revenge. Mr. Stokes forbids further speculation.

Lowen complains that the police will blame servants, not gentry, and openly voices suspicion about Stephen’s arrival coinciding with the murder. Stokes angrily silences him. As Stephen leaves, he notices Stokes watching him with doubt and fear—Stokes is the only person who knows about Stephen’s criminal record and has now connected his arrival with the Viscount’s death.

That evening, Stokes coldly orders Stephen to observe dinner from behind a screen. During the meal, Gilbert loudly asks who murdered the Viscount. Edwin makes a pointed remark about the suspicious deaths of Lettice’s husband and Gilbert’s parents from gastric flu. Offended, Lettice requests funds for Gilbert’s schooling at Eton, which Edwin refuses. Jolyon notes the murderer must still be in the house since the causeway was closed. He points out Professor Müller’s absence from dinner.

The conversation shifts to inheritance and speculation about who stands to receive the estate. Jolyon drunkenly suggests Great-Aunt Decima could be the killer. Edwin considers this seriously, recalling Decima’s violent past and threats against the Viscount, and notes she would have needed a servant’s help. Before Stokes can answer who attended her, Temperance enters with a message: Miss Decima has summoned Stephen to her rooms immediately.

Stephen walks to the Nursery, convinced he will be arrested for murder. Miss Decima demands a full account of the day’s events. Stephen explains the body’s discovery, the fruitless police search, the lockdown, and that everyone suspects him of killing the Viscount. He reveals his fear that his past conviction for attempted murder makes him the perfect suspect. Miss Decima assures him she knows he is innocent—he lacks a murderer’s character.

She argues the murder’s complexity was designed to create chaos and obscure the truth. She proposes they solve the crime together using scientific principles. She will direct the investigation while Stephen acts as her agent—investigating, eavesdropping, and questioning suspects. She warns they must work quickly, and Stephen notes that she appears energized and thrilled by the mystery.

Stephen and Miss Decima plan for hours. Afterward, Stephen goes directly to his room without checking in with Mr. Stokes. An omniscient narrator reveals the murderer—who watched the Viscount die—stands in the study at that moment. A newspaper clipping from Friday, May 20, links King Edward VII’s death to Halley’s Comet.

Friday morning, before Stephen can carry out Miss Decima’s plan, a woman’s scream echoes through the house. He finds Merryn collapsed in the entrance hall, claiming she heard the Viscount’s ghost through a door.

Mr. Stokes intercepts Stephen and takes him to his office. Rather than a reprimand, Stokes gives Stephen black mourning gloves and officially assigns him to attend Miss Decima full-time, replacing Temperance, to keep the difficult old woman under control. Stokes then makes Stephen swear on the Bible that he did not murder the Viscount. Stephen swears his innocence. Stokes believes him. He promises to conceal his criminal past from the Inspector by burning the forged recommendation letter.

As Stephen moves to leave, Mrs. Pearce bursts in: Inspector Jarvis from Scotland Yard has arrived hours early.

The family gathers in the drawing room. Inspector Jarvis makes a theatrical entrance, declaring the Viscount was a close friend and that solving his murder will be his greatest achievement. Jarvis is clumsy and condescending, irritating the servants.

Jarvis proceeds to the study with Stokes, Stephen, Lowen, and Temperance. Stephen senses something is wrong with the room. Stokes reluctantly reveals that during their final phone call, the Viscount reported hearing a scratching sound outside his door. Lowen immediately notes that he found Stephen standing alone outside that door. Stephen explains he was lost and had noticed the missing crossbow bolt. Jarvis has Stephen sit in the dead Viscount’s chair to reconstruct the scene, then announces he will inspect the study alone. As the others file out of the study, Jarvis orders Stephen to remain for questioning.

Alone with Stephen, Inspector Jarvis accuses him of murdering the Viscount and questions why he was hired the day before the death. He theorizes that Stephen forced Miss Decima to provide him with a false alibi.

As Jarvis reaches for the telephone to take Stephen to the police station, Miss Decima herself arrives in her bath chair (a rolling wicker chaise), and reprimands Jarvis, insisting that she required Stephen’s help during the night because she cannot walk and needed assistance during the comet panic. She presents her notebook of comet observations in Stephen’s handwriting as an alibi, challenging Jarvis to verify them with the Royal Observatory. Thwarted, Jarvis shifts his accusation to Miss Decima herself, labeling the murder a crime only a woman could conceive. Miss Decima counters by citing a legal ruling requiring eyewitness evidence before detaining suspects, which stumps him.

Stephen wheels Miss Decima away, leaving Jarvis fuming. Outside, she reveals the legal ruling was fabricated, and the comet readings provide only a weak alibi, but they have bought time. She concludes that Jarvis’s faulty logic will help them—whatever he believes must be wrong.

Miss Decima has Stephen wheel her into the breakfast room, where her sudden appearance terrifies her family. She announces they should expect to see much more of her, then departs. Back in her room, she concludes their fearful reaction is suspicious.

She gives her untouched dinner to Stephen, who is famished, and begins outlining their investigation. Stephen reports that Stokes will protect his secret and tells Miss Decima about sneaking back into the study and being interrupted by Professor Müller.

Decima lists everyone in the house, declaring that one of them must be the murderer. Using collected facts, she eliminates herself, Stephen, and Temperance. She underlines Stokes’s name twice, noting he had the most opportunity, and circles Professor Müller’s name due to his suspicious behavior at the crime scene. She tries to persuade Stephen to seduce Temperance to gain information. Stephen flatly refuses.

Despite his refusal, Stephen attempts to flirt with Temperance in the larder. She hits him with a ham and threatens to report him. He apologizes profusely, which causes Temperance to break down crying about her isolation and misery at Tithe Hall. Trying to make her feel better, Stephen accidentally reveals that someone encouraged him to question her. Temperance correctly guesses Miss Decima is trying to eliminate her as a suspect. To stop her from leaving, Stephen confesses that they plan to solve the murder, revealing the truth about his prison record and explaining that he will be framed unless they find the real killer.

Rather than being frightened, Temperance is thrilled—an avid reader of detective stories, she compares Miss Decima to famous fictional female detectives. She notes Professor Müller’s suspicious behavior in booking travel to Hamburg days before the supposed end of the world, then reveals she has noticed something else odd that could help the investigation.

Stephen brings Temperance to Miss Decima, who is furious that he has revealed their plan. Stephen stands his ground, insisting Temperance is intelligent and refusing to help unless she can join them. Miss Decima agrees to listen.

Temperance reports seeing Lettice hiding something in a drawer. When Miss Decima challenges her to explain the locked-room murder, Temperance proposes it was a mechanical device planted in the study to kill the Viscount after he sealed himself inside—the crossbow outside was a red herring, the scratching sound the Viscount mentioned to Stokes was the device activating, and the killer later removed the evidence. Miss Decima, momentarily speechless, claims this was her next conclusion.

They theorize that Professor Müller, as a scientist, had the means and opportunity to build and plant such a device during the chaos of sealing the house, and declare him their chief suspect. They agree to search the study for evidence when Jarvis leaves. Stephen, Temperance, and Miss Decima officially form an investigative team.

Stephen and Temperance agree to avoid being seen together to prevent rumors of a forbidden romance. Stephen nearly collides with Lowen, who appears frightened and flees up the servants’ staircase. That evening, Professor Müller remains absent from dinner.

Stokes calls Stephen to his office, scolds him for creating a scene in the breakfast room, and warns that Edwin has taken notice of him. He cautions Stephen against relying too much on Miss Decima, noting she has discarded servants before. He expresses guilt, wondering if he could have prevented the Viscount’s death.

Later, while extinguishing lights, Stephen notices Edwin enter the drawing room and—remembering Miss Decima’s advice to keep his eyes open—he eavesdrops. He overhears Edwin telephoning a man named Wilkins and learns that the Viscount changed his will on Thursday—the day he died. Edwin angrily demands to know the changes and whether anyone else has contacted Wilkins, then storms out with a face displaying both rage and fear.