Colette Roberge asks Gamache to provide security at an upcoming controversial lecture by statistics researcher Abigail Robinson.

The university president refuses Gamache's request to cancel Abigail Robinson's lecture over safety concerns.

Abigail Robinson delivers her lecture advocating for euthanasia of vulnerable members of society to a volatile, divided crowd.

A gunman shoots at Abigail Robinson during her lecture, but she is unharmed and the shooter is arrested.

Jean-Guy breaks down and admits to Gamache that he secretly understands some of Abigail's arguments about his disabled daughter Idola.

Gamache and Reine-Marie attend the New Year's Eve party in Three Pines alongside the whole village community.

Abigail arrives at the New Year's Eve party and confronts Vincent Gilbert, who calls her theories morally heinous.

Teenagers discover Debbie Schneider's body lying in the snow in the woods just after midnight.

Gamache and Jean-Guy inform Abigail Robinson that her close friend and assistant Debbie Schneider has been murdered.

Simon Tardif, Edouard's son and a server at the party inn, is revealed as the person who hid the gun at the lecture.

Research in Montreal reveals Vincent Gilbert worked as a young medical student for psychiatrist Ewen Cameron, who conducted cruel experiments on patients.

Gamache shows Vincent Gilbert video footage proving he stood next to Tardif during the shooting and chose not to intervene.

Colette admits she had been secretly working with Vincent Gilbert to dissuade Abigail from spreading her euthanasia theories.

Paul Robinson's hidden suicide letter is found inside a book he gave Colette, revealing he knew who killed Maria.

Abigail taunts Jean-Guy to provoke him into shooting her, but he refuses to be baited and she is arrested.

Chief Inspector Gamache is pulled from his holiday to provide security at a lecture by Abigail Robinson, a statistician publicly advocating for the euthanasia of vulnerable people. When a gunman shoots at Abigail during the lecture, Gamache suspects the attack may have been deliberately staged to boost Abigail's public profile. Days later, Abigail attends a New Year's Eve party in Three Pines where she clashes with humanitarian doctor Vincent Gilbert, and the body of her lifelong friend Debbie Schneider is found murdered in the woods. Gamache and his team investigate multiple suspects, uncovering that Gilbert once worked for a psychiatrist who conducted cruel unethical experiments on patients — giving him powerful motive to silence Abigail permanently. A hidden letter from Abigail's late father reveals he knew who killed his disabled daughter Maria, pointing toward a devastating truth about Abigail herself. After a tense confrontation at Gilbert's isolated cabin, Gamache exposes that Abigail killed her own sister as a child and murdered Debbie to keep that secret buried forever.

The core investigation from Abigail's lecture to the killer's arrest.

Gamache visits Colette Roberge and realizes she may privately agree with some of Abigail Robinson's controversial arguments.

Jean-Guy disobeys Gamache's direct orders and enters the lecture auditorium to hear Abigail Robinson speak for himself.

Haniya Daoud, a Sudanese human rights activist, accuses Gamache of cowardice and hypocrisy for protecting Abigail Robinson.

Gamache learns Colette secretly arranged the lecture and warns her that Abigail may still be in danger from an accomplice.

Gamache meets Abigail Robinson and Debbie Schneider at Colette's home and learns the two women have been childhood friends.

Edouard Tardif insists to investigators that Abigail's message is evil and that no one helped him carry out the shooting.

The Tardif family's motive is confirmed — their elderly mother is severely disabled and would be targeted under Abigail's proposals.

Reine-Marie begins sorting through papers left behind by the late Enid Horton and notices obsessive drawings of monkeys throughout.

Gamache questions Vincent Gilbert about why he concealed the fact that he had attended Abigail Robinson's lecture.

Abigail tells Gamache she had a younger sister named Maria who had a severe disability and died as a child.

Reine-Marie finds a letter confirming Enid Horton was one of Ewen Cameron's patients, co-signed by a young Vincent Gilbert.

Gamache learns that Abigail's mother died by suicide after suffering at the hands of Ewen Cameron, connecting the two storylines.

Investigators searching Debbie Schneider's home find a hidden photograph of Debbie, Abigail, Maria, and Paul Robinson together.

Colette reveals she showed Paul Robinson's farewell letter to both Abigail and Debbie shortly after Paul's funeral.

Vincent Gilbert privately apologizes to Haniya Daoud, and Haniya decides to return to Sudan with a renewed sense of community.

In the days before the murder, Gamache quietly uncovers the personal ties binding the key players together, discovering that Colette arranged the lecture herself and that Abigail and Debbie have been inseparable since childhood. The shooting at the lecture grows more complicated when the Tardif family's motive is revealed — their elderly mother is severely disabled and would be directly targeted by Abigail's policies. Reine-Marie's seemingly unrelated archival work for a local woman named Enid Horton leads to a disturbing discovery: obsessive monkey drawings that eventually expose Vincent Gilbert's long-buried involvement in Ewen Cameron's cruel medical experiments. The investigation deepens when Gamache learns Abigail's own mother was one of Cameron's victims, and investigators find a hidden photograph of the entire Robinson family in Debbie's nightstand. Colette's admission that she showed Paul Robinson's farewell letter to both Abigail and Debbie shortly after his funeral reveals the moment that set the murder in motion years later. The novel closes with Vincent Gilbert offering a private apology to Haniya Daoud, and Haniya returning to Sudan feeling genuinely connected to humanity for the first time.

Key investigations and character revelations surrounding the murder.

Gamache discovers Colette arranged the lecture after Abigail claimed she needed advice while sorting through her late father's papers.

Debbie reveals a surge of political interest followed the shooting, making Gamache suspect the attack was staged to raise Abigail's profile.

Firecrackers at the New Year's Eve party cause Gamache, Jean-Guy, and Haniya to react with alarm while other guests remain calm.

Gamache determines the murder weapon was a log from the bonfire, meaning it was burned and destroyed immediately after the crime.

Abigail and Colette give inconsistent and contradictory accounts of their whereabouts during the narrow window when Debbie was killed.

Alphonse Tardif confesses to hiding the gun at the lecture, then later admits he lied to protect his nephew Simon.

Lacoste realizes Simon Tardif is employed as a server at the inn where the New Year's Eve party was held.

Gamache confronts Vincent Gilbert with documentary evidence that he knowingly participated in Ewen Cameron's unethical experiments on patients.

Abigail admits she came to Quebec not for Colette's support but to blackmail Vincent Gilbert into publicly endorsing her theories.

A coroner unofficially theorizes that Paul Robinson suffocated Maria and staged the death as an accident to cover for someone.

Gamache deduces that Paul Robinson's cryptic letter acknowledged he knew Abigail killed Maria rather than confessing to the act himself.

Gamache takes Colette into custody, theorizing she killed Debbie to prevent Abigail's blackmail scheme from unraveling.

Abigail and Vincent Gilbert slip away undetected to his isolated cabin, forcing Gamache's team to follow urgently by snowmobile.

Gamache lays out his final theory that Abigail killed Maria as a child and murdered Debbie to protect that secret.

Gamache blackmails the government using knowledge of pandemic care-home deaths to ensure Abigail's policies are never implemented.

Gamache grows suspicious early that the lecture itself was orchestrated, and a surge of political interest following the shooting deepens his suspicion that the attack was staged for publicity. When firecrackers at the New Year's Eve party cause only Gamache, Jean-Guy, and Haniya to react with alarm, the mood shifts from festivity to dread — and the discovery that the murder weapon was a bonfire log, already destroyed by the time the body is found, reveals how precisely timed the killing was. A chain of conflicting confessions unravels the truth about the lecture attack: Alphonse Tardif's false admission leads investigators to his nephew Simon, the server who actually hid the gun. Gamache then confronts Gilbert with damning evidence from the Cameron experiments, and Abigail's real reason for visiting Quebec is exposed as blackmail, not friendship. A coroner's unofficial theory about Maria's death and Gamache's careful reinterpretation of Paul Robinson's cryptic letter lead to the real killer — and though Colette is briefly arrested as a decoy, the full truth emerges only at Gilbert's remote cabin, ending with Abigail's arrest and Gamache's quiet act of blackmail against the government to ensure her ideology dies with her public career.