Alain Getty is a home automation engineer who takes a job in the south of France to work on a small camera drone for a technology company owned by Richard Pollock, and moves to a suburb of Toulouse with his wife Bénédicte.
After three months, Richard Pollock invites himself and his wife, Alice, to dinner at Alain’s house. They arrive late and create a volatile scene tied to their marital problems; Alice is rude to Bénédicte. During the evening, Alain discovers a rodent lodged in the S-bend of the kitchen sink, which recovers after he retrieves it. A veterinarian identifies it as a lemming, a Scandinavian animal that does not live in France in the wild.
The next evening Alice visits Alain at his office and reveals that Richard Pollock once tried to kill her, and that the only reason she remains with him is to see him die. She attempts to seduce Alain, who resists but does not tell Bénédicte about the incident. Alice visits Bénédicte alone and alludes to having had sex with Alain, before claiming she feels tired. Bénédicte shows her the guest bedroom, where Alice creates a mess and ultimately shoots herself. Bénédicte feels an urge to spend the night in the guestroom where Alice committed suicide.
Despite Alice’s death, Richard Pollock and Alain embark on a business trip to Biarritz the next day. In the car, Alain asks if Richard had really tried to kill Alice, and Richard asks whether Alain and Alice had intercourse, a question Alain denies.
That evening, Alain calls Bénédicte, who initially doesn’t answer, then abuses him. Concerned, Alain drives home immediately, exhausted. He finds Bénédicte sleeping and unable to wake her. He hears noises from the kitchen and discovers it is full of lemmings; he faints. He awakens in a hospital, learning that he crashed after falling asleep at the wheel and that the previous night’s events were apparently a dream.
Upon release, Alain and Bénédicte visit Richard Pollock’s lakeside holiday chalet. Bénédicte remains cold, and asks why Alain hadn’t told her about Alice’s seduction, repeating Alice’s exact words and insisting that Alain call her “Alice.” They have sex, and Alain falls asleep. When he wakes, Bénédicte and their car are gone. Alain hitches a ride home and discovers that Bénédicte began an affair with Richard while he was in the hospital and is about to move into his house. Alain uses his drone to observe Richard’s house and finds Bénédicte as expected. As Richard and Bénédicte are about to have sex, the camera fails. It appears that Alice has taken control of Bénédicte and resumed her life with Richard inside Bénédicte’s body. Meanwhile, the veterinarian returns the lemming to Alain’s house. The lemming bites Alain and disappears inside the home.
The next day, Richard Pollock confronts Alain at work with pieces of the drone found in the garden. That evening, “Alice-Bénédicte” visits Alain to offer him a deal: if he kills Richard Pollock, he would get Bénédicte back. She gives Alain the key to Richard’s house and leaves.
That night, Alain goes to Richard’s house. He finds Richard and “Alice-Bénédicte” asleep in the bedroom and smothers Richard with a pillow while “Alice-Bénédicte” watches. Alain carries Richard’s body to the kitchen, where he operates the coffee machine and the gas stove. Alice-Bénédicte follows him to the car in a trance-like state. Back at their own house, “Alice-Bénédicte” goes straight to bed. When she wakes, she is Bénédicte again—she says she dreamt of Alice, but remembers nothing of the recent events.
The next day, the couple enjoy time in their garden when they receive news of Richard’s apparent suicide by a gas explosion at his house. Bénédicte asks if he believed Richard killed himself because of Alice’s death, then discovers the dead lemming and discards it. A final voiceover reveals that the neighbour’s son had brought the animal home from a Finland vacation and that the upset father flushed it down the toilet. In the end, Alain mentions that Bénédicte is pregnant.
