Set in Paris, the film follows a chance sexual relationship between a grieving American widower, Paul (played by Marlon Brando), and a young French woman, Jeanne (Maria Schneider). They meet while both are apartment hunting and begin a raw, anonymous sexual affair. Paul insists on keeping their names and personal lives secret, which creates a bubble of intense, detached intimacy.

Jeanne, meanwhile, is engaged to a pretentious young filmmaker and drifts between two worlds — one of reckless passion with Paul and another of youthful idealism. Over time, the emotional stakes rise as the line between detachment and obsession blurs. Eventually, Paul's emotional vulnerability surfaces, and he attempts to form a real connection, leading to a tragic and shocking conclusion.