Based on the 1951 novel The Thin Line by Edward Atiyah, it follows a married businessman who, after killing his mistress, tries to ease his conscience by confessing to his wife and the victim's husband.

Charles Masson, a married advertising executive and father of two children, has an affair with Laura, the wife of his best friend François, an architect.

Unable to stand the strain any longer, Charles announces to Hélène that he will go to the police the next morning to make a confession.

[2] Jacoba Atlas of the Los Angeles Free Press titled Chabrol "a master at showing the macabre murkiness that lies just below the surface of ordinary lives", who, with this film, has made "the definitive statement about the nightmare quality of the cowardly conscience of us all".

[3] David Pirie, writing for the British Time Out magazine, thought equally positive of the film, which he saw as a "tortuous, entertaining study of murder and the expiation of guilt" with "meticulous" direction, acting and script.