Stephen hides the chemicals and rushes outside, passing a smoldering bonfire on the grounds. At the maze entrance, a sign provides a clue for navigating the maze. Solving the wordplay, he takes the right-hand path.

Stephen moves silently through the overgrown maze toward the sound of water. The center is empty—Müller has already passed through. Stephen examines the neglected space: a fountain, a fishpond, and a statue of a weeping woman memorializing Isabella Welt, who died in 1865. He retrieves a curved glass shard from the murky pond water.

Suddenly, Stephen hears footsteps behind him and flees, convinced he is being pursued. He escapes the maze and rounds a corner, colliding with Professor Müller coming from the opposite direction. Müller shoves past him. Stephen realizes someone else was in the maze. Nearby, he overhears Lettice blackmailing Müller again, detailing what she witnessed and threatening to expose him unless he provides money for Gilbert’s schooling.

Stephen returns to Miss Decima’s rooms with the chemicals. Temperance has successfully stolen the crime-scene sketch from Inspector Jarvis. Stephen tells them that Lettice is blackmailing Müller for things unrelated to the murder. When pressed, a mortified Stephen explains that Lettice caught Müller engaging in a sexual act with stolen undergarments and photographs of maids. Miss Decima eliminates both Lettice and Müller from the suspect list.

Stephen describes his other discoveries: the missing fishbowl, Gilbert’s threats, the glass shard in the pond, and being chased by an unknown person through the maze. Miss Decima announces that they must develop the glass negative to compare it with the sketch and identify what changed in the study.

After Temperance leaves, Miss Decima reflects on her sister Isabella, who died giving birth to the Viscount, then announces they will both attend dinner to provoke the suspects.

Stephen wheels Miss Decima into the dining room, shocking the assembled family. Edwin and Mr. Stokes attempt to remove her, but she insists she feels safer with family. Stephen seats her beside Professor Müller.

Miss Decima systematically wreaks havoc: She makes pointed remarks about Müller’s private habits and tricks Jolyon into drinking wine by citing false health benefits. Against Lettice’s wishes, she gives Gilbert her fish. She announces there is an old family scandal—the real one, not the rumor about the Viscount killing his father—that is key to the murder. Edwin freezes visibly. Before he can respond, Müller breaks down and confesses his behavior with the stolen undergarments to the Reverend. Edwin explodes in fury just as Gilbert vomits across the table. Complete pandemonium follows: Jolyon trips over the Professor, the Reverend flees with Müller clinging to him, and Edwin slips in vomit. Miss Decima calmly directs Stephen to wheel her out.

In her rooms, Miss Decima explains the dinner was a calculated experiment: applying pressure to observe reactions. Edwin reacted strongly to her mention of a scandal, while others showed no concern. Having invented the old family scandal to see who would react, she concludes that a genuine buried secret is central to the murder, warning that their investigation has become more dangerous.

When Stephen asks why, she clarifies: The fish at dinner was poisoned and made Gilbert ill. The killer is attempting to murder again.

Above the estate, the comet continues its journey. Below, the murderer plans their next move. A newspaper clipping dated Monday, May 23, 1910, reports that locals believe the recent earthquake and unusual weather are connected to Halley’s Comet, and that a violent storm preceded the seismic event.

Stephen wakes Monday morning from a nightmare, dreading the dangers ahead. On his way to fetch Temperance, he is accosted by a disheveled, sleepless Inspector Jarvis before the assembled household.

Jarvis announces that the postmortem reveals the Viscount died from asphyxiation caused by rotenone poisoning, not the crossbow wound. He theorizes that Stephen hid in the suit of armor, fired a poison-laced bolt through the open study door, and concealed the poison among the Viscount’s chemicals. While listing symptoms from the report, Jarvis mentions necrosis of the gills. Stephen points out the report must concern the fishbowl’s contents, not the Viscount. Professor Müller recognizes the jar and confesses it is his—Conrad had requested it to test on fish. The entire family stares at Jarvis in disgust, and Stephen pointedly excuses himself.

In Miss Decima’s rooms, Temperance is shaken by Gilbert’s severe illness. Miss Decima reasons the poison was intended for another dinner guest, since Gilbert was not supposed to receive fish, and her own attendance was unexpected.

She connects the threads of all the recent mysteries. The fish served at dinner was contaminated with rotenone—highly toxic to fish but only mildly poisonous to humans, enough to sicken a small child. Conrad had poisoned his fishbowl to test the chemical. Müller, fearing he would be implicated, removed the tainted fishbowl and emptied it into the pond, accidentally breaking it and leaving the shard Stephen found. Jolyon, unable to catch fish after Jarvis disturbed the lake, secretly netted the poisoned fish from the maze pond to serve at dinner—he was the man Stephen heard in the maze, fleeing rather than pursuing. She also deduces that Gilbert was the ghost tormenting Merryn.

With these distractions resolved, they can focus on developing the photograph. Miss Decima warns that a storm will soon trap them inside with the killer.

At midday, Stephen and Temperance wheel Miss Decima to a secluded cliff-top heath. They set up equipment to create an enlarged photograph: paper coated with solution is positioned under a device constructed from telescope lenses and a footstool. The glass negative projects the image onto the paper in bright sunlight, where it will develop over the next hour.

While waiting, Miss Decima leads Stephen to a hidden memorial stone she created for her sister, Isabella. This heath was their Star Garden, where they watched the previous appearance of the comet as children. She reflects on their different ambitions and confesses deep regret over their estrangement before Isabella died in childbirth. Pulling out a locket containing Isabella’s silhouette, she reveals that solving the Viscount’s murder is her way to honor her sister’s memory. After requesting privacy, she kisses the locket before concealing it beneath her gown.

The cyanotype is ready. Miss Decima orders Stephen to wash it in the river to fix the image. Despite his fear of water, he descends the steep slope to the waterfall, crosses the steppingstones, and rinses the paper, revealing a crisp blue photograph of the Viscount in his study.

Stephen spots Lowen upstream, running along the river path with a terrified expression. He follows and discovers a tin shed housing the estate’s hydroelectric turbine. Behind it lies a hidden row of trees, each trunk bearing two white painted marks at head height resembling eyes, the wood around them heavily splintered from repeated arrow strikes. He has found a secret target-practice area, suggesting the murder was meticulously planned over an extended period.

Stephen struggles back up the slope and reports his discovery to Miss Decima and Temperance. She concludes the killer must be a long-term resident, not one of the family members who arrived recently.

They lay the developed cyanotype beside the crime-scene sketch to identify differences. After intense scrutiny, they find nothing—the images appear identical. Stephen insists something changed but can only think of the broken teapot. They stare at the images in defeat.

Suddenly, screams pierce the air from the house. They race back across the grounds to find a horrified crowd gathered outside the drawing room. Inside, Inspector Jarvis lies dead beside the fireplace, his skull crushed by the bloody marble bust of Edwin.

The household learns that Merryn discovered the body. Mr. Stokes contacts the police, but the approaching storm has flooded the causeway, trapping everyone on the island. Edwin orders all household members to lock themselves in their rooms until authorities arrive.

Stephen and Miss Decima are isolated in the Nursery as the storm batters the house. Stephen blames himself for Jarvis’s death, convinced he wasted precious time on the study. Miss Decima dismisses his guilt, arguing that Jarvis must have discovered the truth, forcing the killer to silence him. She orders Stephen to search the corpse for the notebook she knows Jarvis carried—and warns him to move carefully, since in this storm no one would hear him scream.

Stephen creeps through the dark, storm-lashed corridors to the drawing room. He retrieves the notebook from Jarvis’s jacket, accidentally dislodging a pair of metal calipers. While replacing them, he notices specks of soot in Jarvis’s hair and on his fingertips.

In the corridor, Mrs. Pearce intercepts him and coerces him into helping in the kitchens. While they work, Stephen asks about the old family scandal. Mrs. Pearce reveals that Mr. Stokes was present when Lawrence Welt, the former viscount, died and has always maintained it was accidental. However, years ago, Conrad had an affair with Miss Decima’s ladies’ maid, Elsie, who became pregnant. After a furious confrontation between Conrad and his father, Miss Decima had Elsie sacked, and Lawrence sent her away, never responding to her desperate letters. The child, Mrs. Pearce notes, would now be approximately 20 years old.

As the kettle boils over, the implications strike Stephen, and he abandons Mrs. Pearce to race upstairs.