Jerry Mulligan (Gene Kelly) is an exuberant American expatriate in Paris trying to make a reputation as a painter. His friend Adam (Oscar Levant) is a struggling concert pianist who is a long time associate of a French singer, Henri Baurel (Georges Guétary). A lonely society woman, Milo Roberts (Nina Foch) takes Jerry under her wing and supports him, but is interested in Jerry more than his art. Jerry remains oblivious to her feelings, and falls in love with Lise (Leslie Caron), a French girl he meets at a restaurant. Lise loves him as well, but she is already in a relationship with Henri, whom she feels indebted to for having saved her family during World War II.
At a raucous masked ball, with everyone in black-and-white costumes, Milo learns that Jerry is not interested in her, Jerry learns that Lise is in love with him, but is marrying Henri the next day, and Henri overhears their conversation. When Henri drives Lise away, Jerry daydreams about being with her all over Paris, his reverie broken by a car horn, the sound of Henri bringing Lise back to him.