In this free adaptation of Alberto Moravia's novel, set in the 1930s, Jean-Louis Trintignant plays a spineless member of the upper class who is psychologically burdened by a sexually traumatic incident in his childhood. Determined to prove his allegiance to the fascist cause, Trintignant's desire is answered when rightist operatives order him to establish contact with a former professor -- now an anti-fascist -- living in Paris. After locating the professor, Trintignant is to report on his movements. Engaged to the devoted, but vacuously bourgeois Stefania Sandrelli, Trintignant decides to combine his mission with a honeymoon.

But on the journey to France, Trintignant is suddenly informed he must kill the professor. When he locates his man, Trintignant's now-lukewarm resolve is further complicated by slinky Dominique Sanda, the professor's wife, who offers herself sexually to Trintignant, then makes advances toward his new bride. Trintignant, under the watchful eye of fascist agent Gastone Moschin, has an increasingly difficult dilemma. His sense of allegiance utterly confused, he takes the path of least resistance.