In Montréal’s back alleys, Amelia Choquet follows a boy through the freezing streets, accompanied by Marc and a group of people with opioid addictions. After an hour of wandering, the boy insists that someone has made dinner for them, and the product is ready. Amelia suspects a setup but feels compelled to continue.

At the Gamache home, Katie Burke and Benedict sit holding hands on the sofa. Gamache tells them the time for lies has passed. Katie explains that she met Bertha Baumgartner while visiting her grandfather at the nursing home. She found Bertha’s insistence on being called the Baroness interesting, since her own grandfather went by the title Baron. Katie introduced them.

Beauvoir reveals that Katie’s mother was born a Kinderoth, making Katie a Kinderoth herself. Katie confirms that her family was raised believing the Baumgartners had cheated them out of their rightful inheritance.

Katie recounts the first meeting between her grandfather, Baron Kinderoth, and Baroness Baumgartner. The Baron presented her with edelweiss flowers, bowed, and called her Baroness. After a long silence laden with over a century of family hatred, she accepted the flowers and addressed him as Baron. Over time, they became inseparable friends and then fell in love.

They planned to marry and merge their estates, ending the conflict and leaving everything to be split equally between both families when they died. However, the Baron died before they could wed. The grieving Baroness attempted to change her will, but the notary refused, believing her not to be legally competent to make that decision. The only change he permitted was to the liquidators. Katie admits that it was her idea to research this possibility and suggest it to the Baroness. The Baroness chose Gamache for his prominence and Myrna because Ruth told her Myrna was a “cleaning woman,” and she thought Myrna would understand her perspective.

Katie reveals that Benedict was named as the third liquidator to act as her proxy, and their breakup was staged to hide the connection. When challenged by Gamache about the elaborate deception, Katie insists that her only goal was to end the destructive family feud that had poisoned generations.

Katie then explains that the Baroness wrote a letter to Anthony, to be given after the will reading, explaining everything and asking him to share any inheritance with the Kinderoths. The Baroness believed Anthony would understand because of his appreciation for the Clara Morrow painting. Gamache and Beauvoir exchange a significant look: No such letter was found.

In Gamache’s study, he calls Lucien on speakerphone, with Beauvoir and Myrna present. Lucien confirms that he found a letter among his father’s files and gave it to Anthony after the will reading. He insists that he did not read it and is dismissive of its relevance to the murder investigation.

Back in the living room, Gamache confronts Katie about the letter’s authorship. Her visible nervousness betrays her before she finally admits that while the Baroness dictated the letter, Katie physically wrote it. Gamache realizes this means there is no proof of the Baroness’s true intentions, as the letter exists only in Katie’s handwriting. Everyone who could verify the story—the Baron, the Baroness, and now Anthony—is dead.

Benedict then reveals another detail from the letter: The Baroness wanted the old farmhouse demolished to help her children make a clean break from the past. He confesses that he went to the farmhouse that night to test a support beam with a sledgehammer, intending to assess the demolition difficulty. He insists that the subsequent collapse was accidental. Katie confirms she asked him to do this.

As Benedict speaks, each person who survived the collapse relives their own terrifying memory: Beauvoir fighting to free himself from those restraining him, Myrna feeling the rubble crushing around her as she grasped Billy’s hand, and Armand remembering Benedict’s body shielding him as the house fell, followed by the sight of Anthony’s hand thrust up through the debris.

Meanwhile, in Montréal, the boy leading Amelia and her desperate group announces that he has found the person they have been seeking.

In the Gamache kitchen, Beauvoir argues that Katie’s entire story is fabricated, and she and Benedict murdered Anthony. Gamache observes that Katie appears to be pregnant, which would strengthen their motive for securing the inheritance. Beauvoir theorizes that Katie falsified the letter to lure Anthony to the farmhouse, where Benedict killed him. Gamache remains unconvinced without concrete proof.

Their discussion turns to the painting of Ruth in Anthony’s house. Beauvoir mentions that Bernard Shaeffer was present when Anthony hung it. He also helped Anthony set up his laptop. Beauvoir recalls seeing a number on the back of the painting, which he assumed was the print number. Gamache shows him one of Clara’s prints, demonstrating that the edition number appears on the front. They realize that the number behind Anthony’s painting must be something else. Beauvoir immediately calls Cloutier, instructing her to try it as the laptop password.

After the guests leave, Gamache receives a call from Dr. Harper, a Montréal coroner. A body has been brought in, and they found Gamache’s contact information on it. Gamache is momentarily terrified it might be Annie or Honoré, but learns it is Anita Facial. Before leaving for the morgue, he takes syringes from a locked desk drawer.

At the morgue, Gamache examines Anita’s body and sees “David 2” written in marker on her forearm. The coroner reveals Anita died of a carfentanil overdose and that six other unhoused people have died the same way in recent days. They examine autopsy photos of the other victims. All have “David” and various numbers written on their left forearms. Gamache realizes these are dosage experiments—someone is testing the carfentanil on human subjects.

Inside the abandoned building, Amelia confronts a young man who reveals that he is not David, saying that she already knows David. The man and his group show their arms, all marked with “David” and dosage numbers. Amelia realizes David is not a person but the street name for the carfentanil itself.

At Sûreté headquarters after midnight, Cloutier tries the number from the painting as Anthony’s password, but it fails. She then shows Beauvoir that Shaeffer has provided access to an offshore account in Lebanon under Anthony’s name, containing over $7 million. Beauvoir suspects that he is stealing separately, and the rest of the money is elsewhere. Looking at the painting again, Beauvoir realizes that the edition number and symbol in the corner might also be part of a code.

Remembering Anita’s message about finding the little girl, Gamache races into the city streets to search for her. He finds a sex worker who saw the girl with Anita earlier. Spotting a flash of red disappearing into a building, Gamache follows and breaks down a locked door. Inside, a man holds the girl with his hand around her throat. When the man refuses to release her, Gamache strikes him, breaking his nose, and carries the girl to safety.

In his car, the silent girl clutches a chocolate bar. Gamache’s phone alerts him that Amelia has located the carfentanil factory. With the tactical team minutes away, he takes the girl to a nearby diner and entrusts her to a server, giving the woman all his money. He then runs toward the factory, calling Beauvoir to inform him of the location and asking Lacoste to retrieve the girl.

Gamache meets the tactical team and is given a protective mask and a firearm. Beauvoir arrives, and Gamache reveals that Amelia Choquet has been working undercover for him all along—no one else, not even Beauvoir, knew. They prepare to raid the factory.

Inside, Amelia has entered a massive lab, where hundreds of workers in protective gear process carfentanil at long tables. The man explains they experimented on people with different doses to determine safe amounts for sale, writing “David” and the dosage on each subject’s arm. He reveals that he knows she is Sûreté cadet Amelia Choquet. Despite her attempts to talk her way out, the man declares her too dangerous to trust and orders her forcibly injected with the carfentanil.

The tactical team storms the factory. Beauvoir, wearing a gas mask, finds Amelia and drags her outside. Gamache immediately injects her with naloxone and begins CPR. A gunman emerges from the factory and aims at them. Beauvoir fires three quick shots, killing him. Medics arrive and take over resuscitation efforts. Gamache watches Beauvoir kneel beside the body of the young man he has just killed.

Past midnight, Gamache and Beauvoir go to Lacoste’s home, where the little girl has been bathed and put to bed. She remains silent, eyes wide with terror, clutching her pillow. Gamache kneels beside her bed and reads from Winnie-the-Pooh in a soft, calm voice. Later, as Beauvoir and Lacoste discuss the case in the living room, they hear Gamache humming “Edelweiss” to the child. Her eyes finally close.

Beauvoir tells Lacoste that Amelia survived and is breathing on her own in the hospital, though brain damage remains a concern. He admits that he had no idea she was working undercover for Gamache. Lacoste tells him she will wait until morning to contact social services about the little girl.

Hours later, Amelia wakes in the hospital, coherent and able to identify Gamache. She is relieved to learn that they secured the carfentanil, and the little girl is safe. Before leaving, Gamache gives her a book of Erasmus for companionship.

At Sûreté headquarters, Cloutier shows Gamache, Beauvoir, and Lacoste what she has discovered on Anthony’s laptop. The number on the back of the painting was an account number for a second offshore account in Singapore. She enters it into the banking portal, revealing an account containing $377 million. Beauvoir orders Shaeffer’s arrest.

The next morning, Beauvoir and Cloutier meet with Hugo and Caroline Baumgartner at Horowitz Investments. Before entering, Beauvoir receives a call from Gamache: The Vienna court has ruled in favor of the Baumgartners. In the meeting, Beauvoir informs them that Shaeffer has been arrested and has admitted to setting up the Lebanese account in Anthony’s name. Hugo, demonstrating his expertise with offshore accounts, notes that they will need the Singapore account number.

Meanwhile, Gamache meets with the Premier of Québec. Following the review board’s decision, he resigns as Chief Superintendent and turns down an offer to return to his old post as head of homicide. The Premier acknowledges that no one, not even Beauvoir, knew about Amelia’s undercover operation. Before leaving, Gamache asks for help with the little girl.

Gamache then visits Benedict and Katie’s basement apartment and informs them that the Vienna court ruled for the Baumgartners. Though disappointed, they accept the decision with resignation.

Back at the meeting, Beauvoir reveals that they found the Singapore account number on the back of the painting. Cloutier brings up the account on the laptop. Caroline reads aloud the account holder’s name: Hugo Baumgartner. She turns to her brother in shock, realizing he is the embezzler. Cloutier arrests Hugo Baumgartner for murder.

A day after the arrests, friends gather at the Gamache home. Ruth and Stephen Horowitz banter on the sofa. Stephen asks Beauvoir to explain Hugo’s crimes. Beauvoir describes how Hugo exploited Anthony’s tarnished reputation by using his brother’s laptop to embezzle money and planting a trail implicating Anthony. He set up the Lebanese account in Anthony’s name as a decoy while hiding the bulk of the money—$377 million—in a Singapore account under his own name. Anthony discovered the fraud last summer but waited until after their mother’s death before confronting Hugo. Myrna suggests that Hugo’s motive was deep jealousy of his handsome, successful, and favored brother.

The critical twist, Beauvoir explains, is that Anthony never found the Singapore account number. Hugo wrote it on the back of the painting for his own safekeeping, not realizing it would become the evidence that exposed him. When Anthony confronted him at the farmhouse, Hugo killed him to protect his secret.

After dinner, Beauvoir and Gamache speak privately. Beauvoir asks why Gamache turned down the offer to return as head of homicide. Gamache explains that the job belongs to Beauvoir now, and he could not be prouder. Beauvoir then confesses that he is leaving the Sûreté, having accepted a position as Head of Strategic Planning for GHS Engineering. Gamache is supportive, understanding that Beauvoir is exhausted by years of death and violence and must prioritize his family. Then, Beauvoir reveals the job is in Paris.

Both Gamache and Reine-Marie are heartbroken but ultimately relieved that their family will be safe and far from danger.

Stephen and Gamache privately discuss how they set up Beauvoir’s new job, although Gamache didn’t know it would be in Paris. Stephen also tells Gamache that his colleagues in Vienna discovered that while the Baumgartner family money is long gone, they owned a building in central Vienna that is real and subject to Nazi-era reparations. If the Baumgartners and Kinderoths file a joint claim, they could inherit tens of millions.

After the party, Ruth invites Stephen to her home. At the bistro, Myrna finds Billy waiting by the fire with two glasses of wine and a pink tulip. She pauses outside to look up at the stars, then steps forward into the warmth.