Gamache directs Cameron to return to his station for flood duties while he and Agent Cloutier head to Montréal.

In Three Pines, Ruth oversees villagers filling sandbags along the river to prevent it from rising into their yards. Children are moved to the local St. Thomas’s Church as the evacuation center. Villagers, including Gabri, Olivier Brulé, Monsieur Béliveau, and Sarah the baker, distribute food and drinks. Billy Williams, who is in love with Myra Landers, offers his sandwich to her. Myra, who doesn’t know about Billy’s feelings, declines, making him believe she is unattainable.

Driving toward Montréal, Gamache struggles not to micromanage the emergency response, reminding himself it is no longer his responsibility. He calls his wife Reine-Marie, who is disappointed to learn he is heading away from Three Pines. Gamache phones Homer to inform him that the search for Vivienne is temporarily suspended. Homer threatens to go to Tracey’s house, but Gamache persuades him not to travel by suggesting that Vivienne might call his home.

On the Champlain Bridge, Gamache fights his vertigo to look over the edge. He sees ice heaving against the pylons and witnesses a frost quake rupture the ice.

At Sûreté headquarters, Gamache leaves Fred with Beauvoir and attends Toussaint’s emergency meeting. Senior officials from the military, the Royal Canadian Mounted Police (RCMP), Hydro-Québec, and Environment Canada are present, along with the Deputy Premier. The chief meteorologist presents an animation showing catastrophic flooding if the northern hydroelectric dams burst.

When others instinctively look to Gamache, he defers to Toussaint. After prompting, however, he proposes digging diversionary runoffs for southern rivers and taking a calculated risk by moving equipment from the northern dams south, where flooding is already occurring. He also advises opening the dam’s sluice gates immediately. The Deputy Premier accuses Gamache of proposing a plan to save his own village, Three Pines. After a heated exchange, Toussaint dismisses Gamache from the meeting.

Gamache collects Fred and decides to take Agent Cloutier to interview Homer. Superintendent Lacoste asks to join them.

Gamache, Lacoste, and Cloutier arrive at Homer’s home in Sainte-Agathe-des-Monts as nearby Lac des Sables breaks up rapidly. Gamache phones in a report about the accelerating thaw. Homer emerges and breaks down upon seeing Fred, his daughter’s dog. He introduces himself and says that her disappearance is all his fault.

Inside, Homer explains that Tracey prevented him from visiting and said Vivienne hated her husband. Homer admits he was not a good father and should have intervened when Vivienne became engaged to Tracey. He reveals that he knew Tracey was beating Vivienne but failed to stop it. On Saturday morning, Vivienne called, saying that she was pregnant and finally planning to leave Tracey. She told him she would arrive Saturday night or Sunday morning and made him promise not to come for her. Homer expresses profound regret for not going to get her.

When asked about a possible lover, Homer angrily insists that Tracey kept Vivienne isolated. He threatens to beat Tracey until he reveals Vivienne’s location and then kill him. To prevent this, Gamache invites Homer to stay with him and Reine-Marie in Three Pines and help organize a search. Homer agrees. Lacoste questions Gamache’s offer, which brings Homer closer to Tracey’s home, but Gamache argues that it is the only way to maintain control.

Clara Morrow stands in her studio, wondering which paintings she would save in an evacuation. The villagers have run out of sandbags and sand. It begins to snow.

Gamache arrives in Three Pines with Homer and Fred. On the way, they stopped in Montréal, where Beauvoir promised to help find Vivienne after the crisis. Homer asked about the northern hydro dams and sensed that Beauvoir’s reassurance was a grim half-truth.

At home, Agent Cloutier drinks wine and feels immense guilt. She reflects that although she is Vivienne’s godmother, she failed to protect her. Searching online, she discovers that Tracey has a website and an active Instagram account to sell his pottery, despite having no internet at home. She calls Superintendent Lacoste and reports her findings. Cloutier has deduced that Tracey has a webmaster named Pauline and might maintain a private Instagram account for secret communication. Lacoste instructs Cloutier to do nothing until morning, but Cloutier disregards this advice.

In Three Pines, villagers gather at Clara’s house. Gamache goes outside to check the sandbag wall, instructing Reine-Marie to keep Homer under watch and away from the car keys. Olivier joins him outside. Leaning over the wall, Gamache discovers an ice jam forming downstream, putting the village in imminent danger. Billy Williams explains he can dig a diversion trench with his backhoe, but the ground must freeze first—sometime after midnight.

Beauvoir calls, reporting that authorities plan to dynamite the St. Lawrence ice jams. He decides to drive to Three Pines before the bridges close.

Ruth organizes sentry duty on the stone bridge. Armand and Reine-Marie take the first shift, watching the river as it breaches its banks. Reine-Marie asks about Vivienne’s case. Armand confirms that Tracey was abusive. They discuss whether using force to extract a confession could ever be justified, with Reine-Marie suggesting that everyone has situational ethics.

At home, Agent Cloutier discovers the IP address for Tracey’s website, determining how he manages it remotely. Agent Cameron conducts an unauthorized stakeout of Tracey’s house, waiting for him to flee.

In Three Pines, Billy Williams periodically tests the frozen ground with a stick to see if it has hardened enough. The temperature plummets, and the sleet stops. Beauvoir’s headlights appear on the hill. After Billy tests again, Reine-Marie tells Gamache the ground has finally frozen solid.

Billy, Gamache, and Reine Marie take a backhoe to Tracey’s farm field, upriver from Three Pines, to dig a diversionary trench. Reine-Marie receives updates from the bridge in Three Pines. Myrna spots the sandbags shifting, and Gabri urgently relays this to Reine-Marie.

A drunken Tracey confronts them, brandishing his .22 rifle and demanding they leave his land. When Gamache creates a diversion by shouting and pointing at the river, Beauvoir disarms Tracey and knocks him down. Tracey breaks free and runs toward the backhoe, screaming at Billy to stop. Billy ignores him, focused on saving the village, and continues digging. The Bella Bella breaks through and pours into the newly dug trench. At the bridge, villagers see the river level drop and cheer with relief.

Gamache suddenly shouts for Billy to stop. The backhoe’s light illuminates the excavated debris, revealing a bright pink duffel bag with a name tag showing the letter V.

Gamache confronts Tracey about the bag. Tracey shows dread but denies knowledge of how it got there. Beauvoir searches the riverbank for Vivienne’s body. Billy suggests the bag came from upriver, near an old logging bridge.

Over Tracey’s objections, they open the bag while Reine-Marie records. Inside are clothing and a bottle of pills, labeled Mifegymiso. They lock Tracey’s unloaded rifle in the trunk and drive to the old logging road with Tracey seated between Billy and Gamache. When the car gets stuck, they proceed on foot.

They discover Vivienne’s car blocking the road. Gamache and Beauvoir find blood smears on the steering wheel and gearshift, and cigarette stubs of Tracey’s brand. They see a missing section of the bridge’s handrail and the churning river 20 feet below. The bridge is too dangerous to search until morning. Gamache gasps in sudden realization and tells Beauvoir that Vivienne is not here, but he knows where she is.