Clara's vernissage opens at Montreal's contemporary art museum, drawing prominent figures from the art world to her first solo exhibition.

The morning after Clara's celebratory party, a body is discovered in the Morrow garden.

The dead woman is identified as Lillian Dyson, an estranged friend from Clara's art school years.

Clara explains to Gamache how Lillian's possessiveness and cruelty gradually destroyed their friendship over many years.

Gamache learns that Lillian Dyson was a feared art critic who deliberately ruined careers with savage reviews.

A cleansing ritual performed in Clara's garden leads to the discovery of a buried Alcoholics Anonymous chip.

At a Montreal AA meeting, Gamache is shocked to recognize Chief Justice Thierry Pineault presiding.

Lillian's AA sponsor Suzanne arrives in Three Pines and finally agrees to speak with Gamache about Lillian.

Gamache brings Lillian's paintings to Sûreté art expert Thérèse Brunel, who finds them remarkably warm and accomplished.

Lacoste discovers that Suzanne Coates, Lillian's own AA sponsor, was the target of Lillian's most savage review.

Gamache questions Suzanne and confirms that Lillian came to Three Pines to make amends through AA's ninth step.

Beauvoir, under the influence of narcotics, furiously confronts Gamache and accuses him of abandoning him to die.

At a dinner party at the Morrow home, Castonguay attempts to strangle Marois over a stolen client.

Gamache names Denis Fortin as Lillian Dyson's killer before the assembled guests at Clara's dinner party.

Clara asks Peter to leave the village for a year's separation after the case is resolved.

A Trick of the Light opens with Clara Morrow's long-awaited solo art exhibition in Montreal, a triumph immediately overshadowed when a body is found in her garden the following morning. The victim is identified as Lillian Dyson, an estranged friend whose jealousy once nearly destroyed Clara's creative confidence and career. Gamache and Beauvoir pursue leads through the secretive world of Montreal's art dealers and the recovering community of Alcoholics Anonymous, where Lillian had quietly transformed herself before her death. As Gamache gathers suspects at Clara's dinner table, old rivalries and hidden resentments erupt into violence, and the truth behind Lillian's presence in the village is finally exposed. Denis Fortin, a gallery owner whose artistic ambitions Lillian once crushed with a cruel review, is revealed as the killer — a man who used Lillian's own attempt at redemption to set a trap for her. The case's resolution forces Clara to confront the damage jealousy has done inside her own home and to ask her husband to leave.

The murder investigation from body to killer.

Beauvoir poses as a Le Monde art critic at the vernissage, using overheard artistic terms he doesn't understand.

Gamache offers Olivier a handshake at the Morrow garden crime scene, beginning the repair of their fractured friendship.

The coroner tells Gamache that Lillian's broken neck indicates the killer harbored deep personal hatred toward her.

Peter confesses to Gamache that he voiced criticisms of Clara's art to Lillian, which Lillian then published.

Marois and Castonguay both arrive in Three Pines and set up a workspace to compete for the Morrows.

Lillian's car is found parked far from the party, filled with maps for a village that appears on no map.

Myrna reads Clara the glowing critical reviews of her show aloud in the bookstore to help her truly celebrate.

Gamache and Beauvoir find Lillian's apartment walls covered in warm, optimistic paintings of Montreal rendered as a forest.

Gamache visits Denis Fortin's Montreal gallery and learns that Fortin accepted Clara's invitation to her party.

Clara discovers that Peter deliberately hid congratulatory business calls from her following her successful show.

Annie tells her parents she needs to speak with them seriously, hinting that her marriage is in trouble.

Chief Justice Pineault introduces Gamache and Beauvoir to Lillian's sponsor Suzanne at a local Tim Horton's.

Gamache assigns Lacoste to search newspaper archives for the identity of the target of Lillian's cruelest review.

Fortin admits he directed Lillian to Clara's party and staged the murder at her home to destroy her triumph.

Rosa the duck returns to Ruth, ending the elderly poet's long daily vigil at the edge of the village.

While Gamache pursues the main line of inquiry, a series of secondary events reveals the hidden landscape beneath the art world's polished surface. Witnesses lie, evidence is concealed, and the personal lives of both suspects and investigators are quietly unraveling. Peter's jealousy becomes measurable in small, deliberate cruelties, while the competing ambitions of gallery owners Marois and Castonguay expose how deeply resentment can fester in creative circles. Gamache's visit to Fortin's gallery and the discovery of Lillian's warmly painted apartment offer contradictory portraits of the same woman — one a feared destroyer, the other a person genuinely trying to change. The investigation's emotional stakes are raised when Gamache's own family is drawn in, and Fortin's final confession explains not just the killing but the cold calculation behind its location. The pool closes with a moment of quiet joy as Ruth's beloved duck returns home, offering a small but genuine promise of renewal.

The investigation's key secondary discoveries and complications.

Marois examines Ruth's portrait and suggests the old woman's expression of hope may be merely a trick of the light.

Peter retreats alone to his studio, bitter and resentful as phone calls celebrating Clara's success pour in.

Clara admits to Myrna she feels deep resentment more than grief over the unexpected death of Lillian.

Artists Normand and Paulette reveal to Gamache that gallery owner Denis Fortin attended Clara's vernissage.

Marois and Castonguay admit to Gamache they both recognized Lillian and had reviewed her new portfolio of paintings.

Ruth tells Beauvoir she has deduced he is in love with Annie Gamache and urges him to act on it.

Gamache finds Lillian's AA Blue Book and meeting schedule while searching her Montreal apartment for clues.

Peter admits to Clara that her faith and hope in humanity are what he truly resents, not just her art.

Suzanne appears uninvited in Clara's garden, claiming she had to see the place where Lillian died.

Gamache and Thérèse Brunel reluctantly discuss the leaked warehouse raid video, agreeing the investigation appears corrupt.

Lacoste reads a second devastating Dyson review in the archives and feels she is watching someone's artistic death.

Gamache finds Beauvoir's supply of OxyContin and firmly orders him to return to counseling and seek help.

Gamache apologizes directly to Olivier for wrongfully arresting him and suggests they both deserve to heal.

Pineault reveals to Gamache that Brian is actually his AA sponsor, correcting Gamache's mistaken assumption entirely.

Beauvoir takes a pill before finally calling Annie to invite her to dinner for the first time.

Beneath the murder plot, A Trick of the Light traces a series of quieter but decisive moments in which characters discover uncomfortable truths about themselves and each other. From Marois's skeptical reading of Clara's portrait at the opening vernissage to Ruth's sharp insight into Beauvoir's feelings, the novel turns on moments of recognition — some healing, some devastating. Peter's confession to Clara about his jealousy and Gamache's discovery of Beauvoir's addiction mark the points at which denial can no longer hold. The investigation's emotional core is laid bare when Gamache and Brunel face the conspiracy behind the leaked raid video, and when Lacoste reads Lillian's second review and feels the full cruelty of what a critic's pen can destroy. The pool closes with two quietly significant reversals: Pineault correcting Gamache's assumption about Brian, and Beauvoir taking a pill before making the call that may begin his long road back.