Jerry Mulligan is an American soldier living in Paris and trying to make it as an artist. He's friends with another expat, Adam Cook, who ekes out a living as a concert pianist. They're both broke, but happy.
Adam meets his friend, the French singer Henri Baurel, at a café. Henri tells Adam all about his girlfriend, who's beautiful and complicated. When Jerry stops by, Adam introduces Henri to Jerry. Then Jerry heads out to try selling his paintings on the streets of Paris.
Jerry meets Milo Roberts, yet another American living in Paris. She's a wealthy art aficionado, and she digs Jerry's artwork. Milo offers to be Jerry's sponsor; she'll use her connections to help him make a name for himself in the art world. We soon get a hunch that she wants more than just Jerry's business.
At the club, Jerry also meets Lise Bouvier. He's instantly smitten, not knowing that she's Henri's girlfriend. Lise, meanwhile, has absolutely zero interest in Jerry. Milo gets mad at Jerry for hitting on Lise in front of her and leaves in a huff.
The next morning, Jerry and Milo make up over breakfast. Milo still wants to help make him an art world all-star. After breakfast, Jerry goes to the perfume shop where Lise works. She's not at all happy to see him but ultimately agrees to meet him later that night.
Jerry and Lise rendezvous at a café, but Lise quickly suggests that they go somewhere more private. Jerry and Lise dance along the River Seine. Then they kiss. Then Lise runs away. Hello, love triangle! Meanwhile, Adam has a dream that he's performing Gershwin's "Concert in F" to rave reviews all by himself—as in, he's not just the pianist, but also the conductor, the rest of the orchestra, and even the audience.
Milo sets Jerry up with a huge art studio and tells him that she's scheduled an exhibition for him in three months. Jerry initially turns down both the studio and the exhibition, but eventually accepts both and gets to work, churning out canvas after canvas.
Jerry and Lise keep dating, and Lise keeps mysteriously dashing off. Jerry complains to Adam about Lise's shady behavior, and Adam realizes that Jerry and Henri are both in love with the same woman. Henri shows up and tells Jerry that, if he really loves this girl, he should tell her, not knowing that this girl is his beloved Lise.
When Jerry and Lise meet again on the Seine, Jerry tells Lise that he loves her, and Lise tells Jerry that she's engaged to Henri. Not only that, but they're leaving for America tomorrow. Jerry's crushed. Lise tells him that she loves him, and then Jerry leaves. Still reeling from Lise's announcement, Jerry turns up at Milo's hotel room, kisses her, and invites her to the art students' ball. Milo's stoked.
At the art students' ball, Adam tells Milo that Jerry's just not that into her, and Henri overhears Jerry and Lise's emotional goodbye. Henri and Lise leave, presumably forever, and Jerry imagines himself and Lise performing an elaborate ballet together inspired by a series of famous French painters.
When Jerry's daydream ends, he hears a car horn. It's Henri and Lise. Lise kisses Henri goodbye, and he drives off again. Jerry rushes down to meet Lise in front of the building, and then they walk off into the early morning together, hand-in-hand.