One night while on vacation, Jean Fournier (Claude Mann), a bank employee, is introduced to gambling by a friend and soon becomes hooked. Intoxicated by winning large amounts of money, Jean abandons his normal middle-class existence and moves to the South of France where he meets Jackie (Jeanne Moreau), a compulsive gambler. Jean is fascinated by Jackie's insolent beauty and a feeling of complicity is soon established between them - but there is also a misunderstanding. Jean is sincerely in love with Jackie, whereas Jackie only puts up with Jean out of superstition; she believes that he brings her luck.
Together, they win and lose considerable sums at the table, but Jean soon grows tired of this rootless, chaotic life, as well of being the "mascot" of Jackie, for whom it seems that life itself is just a game of chance.