David is escorted to a hotel after his wife has left him for another man. The hotel manager reveals that single people have 45 days to find a partner, or they will be transformed into an animal of their choice; the dog accompanying David is his brother. David chooses to become a lobster. David makes acquaintances with Robert, a man with a lisp, and John, a man with a limp, who become his quasi-friends.
The hotel has many rules and rituals: masturbation is banned, but sexual stimulation by the hotel maid is mandatory, and guests attend dances and watch propaganda extolling the advantages of partnership. Residents can extend their deadline by hunting and tranquilizing the single people who live in the forest; each captured “loner” earns them a day. On one hunt, a woman with a fondness for biscuits offers David sexual favours, which he declines. She tells him that if she fails to find a mate, she will kill herself by jumping from a hotel window.
John wins the affections of a woman with constant nosebleeds by purposely smashing his nose in secret. They move to the couples section to begin a month-long trial partnership. David later decides to court a notoriously cruel woman who has tranquilized more loners than anyone else. Their initial conversation is accompanied by the screams of the biscuit-loving woman, who has injured herself by jumping from a first floor window. David pretends to enjoy the woman’s suffering to gain the heartless woman’s interest. He later joins her in a jacuzzi where she feigns choking to test him. Noticing that he makes no attempt to help her, she decides that they are a match, and the two are shifted to the couples’ suite. David wakes up one morning and finds that she has kicked his brother to death. As David cries in response to losing his canine sibling, she concludes that their relationship is a lie and attempts to drag him to the hotel manager to have him punished. However, he escapes and, with the help of a sympathetic maid (who is later revealed to be a mole working for the loners), tranquilizes his partner and transforms her into an unspecified animal.
David escapes the hotel and joins the loners in the woods. In contrast to the hotel, they forbid any kind of romance, which is punishable by mutilation. David, who is short-sighted, begins a secret relationship with a woman who is also short-sighted. They develop a gestural language they use to communicate. They are taken on covert missions to the nearby city, where their cover requires them to appear as lovers, which they secretly enjoy.
The loners launch a raid to sabotage the work of the hotel. David tells the woman with nosebleeds that John has been faking his. Other loners hold the hotel manager and her husband at gunpoint, tricking him into shooting his wife to save himself, but the gun is not loaded. They leave the couple to face each other.
The leader of the loners obtains the short-sighted woman’s journal and discovers her plan to escape with David. The leader takes the woman to the city, ostensibly to have an operation to cure her short-sightedness, but instead has her blinded. David and the woman try to find something else that they have in common, to no avail. One morning, David overpowers the leader, leaving her tied up in an open grave to be eaten by wild dogs. He and the blind woman escape to the city and stop at a restaurant. David goes to the restroom and hesitantly prepares to blind himself with a steak knife.