Stephen wheels Miss Decima to the study. She orders him to recreate the conditions of his entry by turning off the lights, then turns them on after a few seconds and has him describe what he saw. She asks what he did not see and prompts him to compare the room with the cyanotype they developed on the heath. Stephen realizes the mirror above the mantelpiece is missing. Miss Decima explains Professor Müller removed it before photographing the room to avoid glare, and Stephen deduces the murderer removed it. Recalling Mr. Stokes dropping the teapot at the murder scene, Stephen experiences a moment of complete realization. At that instant, all the lights in the house go out.

Miss Decima deduces the murderer has cut the power. Stephen’s plan to use the lift fails due to the outage, and he finds Mr. Stokes handing out candles in the main stairwell. He urgently instructs Temperance to run to the Nursery, lock the doors, and barricade herself inside. He then leads Miss Decima down the servants’ staircase, only to find the Nursery doors and side exit open. Through the rain, Stephen sees a man dragging a screaming Temperance into the forest.

Stephen deposits Miss Decima in her bedroom, grabs a caulking iron, and runs into the storm. At the bridge over the waterfall, he spots a hunched figure in the ferns. Believing it to be Lowen—Conrad’s abandoned son seeking revenge for his mother Elsie—he leaps at the figure, only to discover it is Lowen’s corpse. Temperance appears behind him, corrects him that it was a daughter, and strikes him on the head with a hammer.

Stephen regains consciousness, bound hand and foot. Temperance reveals she is Conrad’s illegitimate daughter by Elsie, whose life Conrad destroyed. Her plan for revenge was perfect until Stephen and Miss Decima interfered. She admits Lowen’s death was unplanned—they killed him after he discovered their target-practice site—and that her accomplice is currently killing Miss Decima. On the bridge, she stuffs rocks into Stephen’s pockets and outlines her plan to frame him for all the murders and stage his drowning as a death by suicide, then pushes him into the raging river.

The river batters Stephen toward the hydroelectric turbine’s wheel. At the last second, he grabs a bar across the sluice gate and hauls himself onto the stone slipway, his arm broken. He unties his feet, limps back through the forest, and retrieves his caulking iron. Approaching the house, he creeps into Miss Decima’s bedroom to find her holding a lit candle and tea caddy while Mr. Stokes aims a crossbow at her throat. Temperance stands nearby, shocked.

Miss Decima threatens to drop her candle into the caddy, which she claims contains explosives. Stephen accuses Stokes of letting Conrad die, and Stokes’s silence confirms it. Miss Decima prompts him to explain Lawrence Welt’s death. Stokes confesses his drunken father accidentally shot Lawrence with a crossbow. Conrad prevented Stokes from getting help, watched his father die, then used the incident to blackmail both men for 20 years. Temperance later recruited the guilt-ridden butler as her accomplice.

Stephen reconstructs the murder: Stokes planned to spend the night in the study, hence two oxygen tanks. The bonfire was his failed attempt to destroy the charred crossbow. Stephen’s arrival as an ex-convict provided a scapegoat. Temperance climbed from her loosely sealed window onto the roof, shot Conrad through the chimney, and Stokes let him die and cleaned the scene. Temperance faked the morning’s events. The crucial element was Stokes hiding in the dark study until the door was broken, then slipping out unnoticed—the mirror removed so Stephen would not catch his reflection. The cold tea, made the previous night, was a final clue.

Stokes praises Stephen’s deductions and admits he could not let him take the blame. Miss Decima clarifies that Jarvis’s death was accidental, though Stephen notes Jarvis had solved the murder method. Stokes hands the crossbow to Temperance and asks Stephen to let him pass. On Miss Decima’s nod, Stephen steps aside.

Miss Decima offers Temperance safe passage. Stephen, faint from blood loss, protests but is helpless. He asks if Temperance ever liked him; she looks at him with pity and surprise, says she is sorry, and leaves. Miss Decima reveals the explosive was a bluff. Stephen faints.

A newspaper article later reports that Mr. Matthew Stokes died by suicide, leaving a confession taking the blame for the murders. Temperance Atkins is missing and sought by police.

Days later, Stephen confronts Miss Decima for letting Stokes die and Temperance escape. She argues it was Stokes’s choice and that she let Temperance go to protect Stephen, insisting she views him as a friend. Stephen laments that the killer escaped and the family remains. Miss Decima uses Comet Halley as an analogy for inevitable change, however slow. When the family returns from the funeral, she insists on sitting with Stephen at the back among the servants for the will reading.

The solicitor reads Conrad’s will, which gives minimal bequests to staff and insulting gifts to his cousins Edwin, Jolyon, and Lettice. The bulk of the estate will support the Cometary Cataclysm Society under Professor Müller, but control must remain within the family. As no other relatives showed interest in science, the will bequeaths everything—Tithe Hall, World’s End, the fortune, and control of the Society—to Miss Decima Stockingham, on the condition that she find and educate a scientifically minded successor. The family is furious. Miss Decima has Stephen wheel her to the front, where she begins reading a list of people who must leave immediately.

With the family gone, Miss Decima outlines her plans: fighting Edwin’s legal challenge, buying Lettice’s loyalty by offering to send Gilbert to Eton, and letting Jolyon self-destruct. She will fund a science scholarship for young women, naming Clemency Wellbeloved as the first recipient and admitting she judged her too hastily and will ensure all staff remain employed in memory of Mr. Stokes. She then awkwardly reveals she is moving to London and offers Stephen a position as her personal attendant and partner in a new detective agency. Stephen joyfully accepts. She tells him he can now call her Decima. They sit together, watching the comet.

In June 1910, Stephen and Decima settle into Conrad’s London apartments, where she establishes her detective agency. Temperance is never seen again. Edwin dies in 1913 after contesting the will. Professor Müller dies in 1914 during a public demonstration of his own chemical weapon. Jolyon is discharged from the Navy and dies in poverty in 1917. Lettice marries multiple times and dies in 1938. Gilbert changes his name and becomes Home Secretary. The 1910 comet panic is forgotten; scientists later determine the tail missed Earth by nearly 200,000 miles. Comet Halley continues its elliptical orbit, returning in 1986 and again in 2061.