Jacqueline and Jacques, newlyweds, are in frequent discussions. The cause is Jacqueline's flirtatious nature, which she has not been able to shake off despite her marriage. One of her tennis partners, Pierre, declares his love for her, but Jacqueline advises him to marry Simone instead. Meanwhile, the husband is approached by an unsympathetic character who suggests that Jacqueline is his mistress.
Vincenzo, a railwayman who works on the Rome-to-Paris line, has a wife in Italy and a mistress in France: Ginette, a widow with a four-year-old daughter. When he's offered a job in Paris he decides to move, but unfortunately his wife and petulant brother-in-law insist on accompanying him to Paris, and he must decide between the two women in his life.
To conceal his nighttime infidelity from his wife Gisele, Pierre calls in a particularly shady private detective to concoct an alibi. He is introduced to a certain Robert Crémieux, who will pass himself off to Gisèle as a childhood friend with whom he spent the night until the early hours of the morning. The so-called childhood friend turns out to be a delinquent accused of burglary and rape on the very night he was supposed to be with Pierre. Pierre is immediately suspected of being his accomplice and, trapped by his own turpitude, arrested. Entangled in his lies and denials, he has to admit his infidelity to get out of the impasse.
