Tomas, a German filmmaker, and Martin, an English printer, are a married couple residing in Paris. One evening at a party, Tomas sleeps with a friend, a young teacher named Agathe; this encounter blossoms into an affair that despairs Martin. After trying and failing to reconcile with Martin, Tomas moves in with Agathe. Even though Tomas disagrees, Martin begins plans to sell their countryside house. He also begins a relationship with a writer, Ahmad, which makes Tomas jealous; Tomas continues to make overtures towards Martin and the two eventually sleep together. After this, Tomas informs Martin that Agathe is pregnant. Martin is upset but attempts to take it in stride as he has always wanted children. Meanwhile, Tomas meets Agathe's parents, who disapprove of him. Martin and Tomas host Agathe and some of their friends at their house. Agathe overhears Martin and Tomas having sex and breaks up with Tomas. Martin and Tomas reconcile and make plans to visit Venice together; before they leave, Agathe informs Martin that she has had an abortion and Tomas has kept this from him. Martin breaks up with Tomas for good. Tomas visits Agathe's school and begs for her to return to him, but she rejects him. A heartbroken Tomas rides his bike through the streets of Paris.