L'Amant Double follows Chloé, a young, fragile woman who falls in love with her psychoanalyst, Paul. The couple moves in together, but Chloé soon discovers that her lover is concealing a part of his identity. She learns that Paul has a twin brother, Louis, who is also a psychoanalyst but with a contrasting personality. Drawn to Louis, Chloé becomes entangled in a web of passion and deceit, blurring the boundaries between reality and illusion as she struggles to reconcile her desires with the dark truths she uncovers.