Maurice Legrand, a meek and unhappily married cashier, encounters Lulu, a beautiful young woman involved with the unscrupulous pimp Dédé. Falling deeply in love with Lulu, Maurice begins an affair with her, unaware that she is manipulating him under Dédé's instructions. As he gets more entangled in Lulu's web of deceit, Maurice supports her financially, but his actions raise suspicions among his colleagues and wife.

Despite his growing attachment, Maurice starts to notice the inconsistencies in Lulu's behavior. His downward spiral intensifies as Lulu and Dédé's true motives become clearer. Misled by love and driven by desperation, Maurice's attempts to win Lulu's affection and avoid discovery ultimately lead to unforeseen consequences. His initial escapism from a stifling marriage transforms into a tragic entanglement that threatens to destroy everything he holds dear.