Gamache responds to a mass shooting at École Polytechnique and provides first aid to a young female student named Nathalie Provost.

Gamache and young agent Jean-Guy Beauvoir retrieve Clotilde Arsenault's body from a remote lake after her two children report her missing.

Gamache discovers that Clotilde sold access to her own children for sex, with corrupt police officials among the paying clients.

Gamache confronts corrupt Captain Dagenais and is surrounded at gunpoint by a group of armed agents loyal to the captain.

Gamache tells Fiona and Sam he knows they killed their mother; Sam confesses and claims Fiona planned and led the killing.

Fiona is tried as an adult and sentenced to fifteen years in prison, with Sam's damning testimony partly responsible for the harsh outcome.

Harriet suggests a hidden room may exist above Myrna's loft, and Billy produces an old letter describing a sealed room in Three Pines.

Billy knocks down a bricked-up wall in Myrna's loft, revealing a hidden room that contains a large and mysterious painting.

Gamache and Billy visit the Godin house and learn that Patricia Godin died shortly after forwarding the Pierre Stone letter to Billy.

Gamache opens a full homicide investigation into Patricia Godin's death after autopsy evidence suggests she was strangled and then staged to look like a suicide.

The shorthand symbols etched across the painting and the elephant are decoded, revealing the same threatening phrase repeated in multiple languages.

Gamache rushes to the maximum-security prison and discovers that Fleming has been impersonated by a substitute for an unknown length of time.

Gamache reveals to Agent Moel that Fleming has escaped and that the painting conceals the faces of all seven of Fleming's murder victims.

Fleming drops his Mongeau disguise, holds Reine-Marie at knifepoint, and forces Gamache back to his home to carry out the planned killings.

Fiona defects to guide Amelia and Harriet to the house; Amelia shoots Sam and Gamache kills Fleming with a loose brick in the basement.

The story opens with two defining moments from Gamache's past: his response to the Polytechnique massacre and his investigation into the murder of Clotilde Arsenault, whose children were sold for sex by their own mother with corrupt police among the clients. After the killers are brought to justice and Fiona is imprisoned, the narrative jumps to the present, where the discovery of a hidden room in Three Pines triggers a chain of events that leads Gamache to suspect that the serial killer Fleming has escaped from prison. Fleming has spent years engineering an elaborate revenge plot, weaving together a mysterious painting, a fabricated letter, and a staged suicide to lure Gamache toward a fatal confrontation. As decoded messages and hidden clues point unmistakably to Fleming, Gamache races to protect his family while the killer closes in, eventually revealing himself by taking Reine-Marie hostage. The confrontation ends in Gamache's own basement, where he kills Fleming with a brick — the same object that began the whole mystery — while Fiona's last-minute defection proves decisive in saving the family.

Gamache's investigation from old wounds to Fleming's final defeat.

After witnessing the Polytechnique shooting, Gamache withdraws his application to the Intelligence division and applies to homicide investigation instead.

Gamache descends to the basement of the police station and specifically selects the discontented, overlooked young Jean-Guy to assist with the Arsenault murder case.

Clotilde's car is found with blood evidence, confirming it was used to transport her body to the lake after she was killed.

Sam is found badly beaten in an alley after the two children run away, and he refuses to identify who attacked him.

Harriet presents Myrna with a factory-original brick as a sentimental graduation gift and accidentally reveals that Myrna is considering leaving Three Pines.

Fiona and Harriet receive their engineering degrees at a solemn graduation ceremony at École Polytechnique, with Nathalie Provost also in attendance.

Reine-Marie inspects the book found in the sealed room and identifies it as a grimoire belonging to 17th-century accused witch Anne Lamarque.

The bronze elephant from the sealed room is traced to Olivier's inn, where it vanished after a guest using the name Lillian Mountweazel stayed.

Gamache sends Reine-Marie to England under the pretense of researching the original Paston Treasure, but his true motive is getting her away from danger.

Jean-Guy accidentally glimpses Sam's phone and sees photographs taken inside Gamache's bedroom, confirming that someone has secretly entered the house.

Art curator Dr. Louissaint confirms the painting was likely created in a prison, where inmates once participated in a large-scale art reproduction project.

Jean-Guy finds a used exhibition ticket for a show featuring the Paston Treasure tucked inside a book of poems at the Godin house.

Sylvie Mongeau dies unexpectedly overnight, and Gamache orders an official examination of her body despite Robert Mongeau's insistence she died of cancer.

Harriet regains consciousness to find herself tied to a tree deep in the woods, alone and far from the village.

Reine-Marie travels to visit Fiona in prison, implying she offers forgiveness for Fiona's role in the plot against the Gamache family.

This pool traces the quieter but essential moments that give the main mystery its texture and depth. It begins with the personal evolution of Gamache, who redirects his career after the Polytechnique tragedy, and follows the early bonds forged during the Arsenault case, including Gamache's deliberate decision to elevate the discontented young Jean-Guy from obscurity. In the present day, the pool tracks life in Three Pines as it is upended by the hidden room, following the evidence trail from a stolen bronze elephant to an ancient grimoire to a telltale exhibition ticket found at the Godin house. The investigation broadens to include Sylvie Mongeau's suspicious overnight death and Harriet's terrifying ordeal tied to a tree in the woods, both of which raise the stakes as Fleming's threat grows. The pool ends with a quiet act of grace, as Reine-Marie chooses forgiveness and visits Fiona in prison after everything the young woman helped set in motion.

The supporting threads, clues, and characters woven through the investigation.

Gamache recognizes a senior police official in the partially undeleted files on Clotilde's computer, revealing that Dagenais returned to the house to destroy evidence.

Gamache follows a fleeing agent to Dagenais's home and, after a struggle, secures a notebook listing all of Clotilde's paying clients by name.

Gamache delays his meeting with Sam and Fiona by one day to wait for Jean-Guy; during that night the two children escape, a decision Gamache considers disastrous.

Fiona secretly allows Sam into the Gamache home to snoop while Gamache and Reine-Marie are away, despite Gamache having expressly forbidden Sam from entering.

Gamache finds an iron ring on his basement floor and concludes it likely belonged to one of Fleming's murdered victims, meaning Fleming has been inside his home.

Clara examines the painting and the group realizes it incorporates modern objects that do not appear anywhere in the original 17th-century Paston Treasure.

Gamache deduces that the painting was placed inside the hidden room during the specific period when both he and Myrna were out of town.

Gamache assigns Amelia Choquet to go undercover near Sam, then immediately cancels the plan after suspecting Fiona may have overheard the conversation.

Ruth identifies the symbols etched across the painting and carved into the elephant as shorthand notation, finally allowing the hidden messages to be read.

Gamache sits alone in front of the painting late at night and arrives at a terrifying conclusion about its creator, leaving Jean-Guy visibly alarmed.

Gamache discovers that Fleming's prison residence timeline overlaps with Clotilde Arsenault's location at the time of her murder, suggesting Fleming could be Fiona's biological father.

After Gamache physically threatens the uncooperative warden during a furious confrontation, the warden is arrested for conspiring in Fleming's escape from prison.

DNA results appear to confirm that Fiona is the biological daughter of Claude Boisfranc, seemingly identifying which man in Three Pines is Fleming in disguise.

Amelia finds Harriet fleeing through the woods and discovers Boisfranc's mutilated body nearby, realizing Fleming's true identity is still completely unknown.

After the crisis ends, Gamache reveals to Reine-Marie that Amelia is the daughter of the drunk driver who killed his parents, making their bond all the more profound.

This pool focuses on the moments where Gamache's understanding of events shifts, often dramatically, changing the direction of both past and present investigations. It opens with the discoveries that crack the Arsenault case open — recognizing Dagenais in the files, securing the damning client notebook, and making the one-day delay that allows the children to escape — a decision Gamache will long regret. In the present, the pool tracks a series of escalating realizations: the secret intrusion into Gamache's home, the painting's hidden clues, and the growing terror that Fleming may be concealed in plain sight within Three Pines itself. As more pieces connect — the shorthand decoded, a disturbing prison timeline, a DNA result that seems definitive — Gamache is forced to question every assumption he has made about who is dangerous and who can be trusted. The pool captures the cascade of reversals in the final stretch, including a DNA result that deliberately misleads and a body in the woods that arrives too late to help. It ends with Gamache sharing the most personal revelation of all: the hidden connection between Amelia Choquet and the accident that first set the course of his life.