Diamonds Are Brittle (1965)

Bernard Noblet is a modest bank clerk with a passion for billiards. His fiancée Juliette, a schoolteacher, dreams of one day living the high life, and his best friend Roger, a little-known inventor, is equally despairing. After careful consideration, a solution emerges: a cleverly organized hold-up will enable them to live in luxury for the rest of their lives. Bernard soon makes the acquaintance of Bettina and her mother, Madame Ralton, professionals in the "heist" business.