Based on the 1962 novel The Long Saturday Night by American author Charles Williams, it tells the story of Julien Vercel (Jean-Louis Trintignant), an estate agent who is suspected of murdering his wife and her lover.

In an effort to prove his innocence he wants to go to Nice, where his wife previously worked, but his secretary, Barbara Becker, argues that she should do the research on his behalf.

While Julien hides in his real estate office instead of surrendering to the police, Barbara, who travels to Nice, starts investigating the past life of Marie-Christine.

She learns that Marie-Christine's real name was Josiane Kerbel, that she has been married to a gambler, and that she lost a great deal of money gambling on horses.

At the theatre, a box office clerk and former lover of Massoulier's, who accused Julien of murder on the phone, is stabbed to death.

Clement, who smells the trap based on the sound recording at the last minute, commits suicide in a telephone booth after having admitted everything on the phone, when he sees the approaching cops.