In 1938 Paris, Marcello Clerici finalizes his preparations to assassinate his former college professor, Luca Quadri, leaving his wife Giulia in their hotel room. After receiving a call, Marcello is picked up in a car driven by his subordinate, Special Agent Manganiello.

A series of flashbacks depict Marcello discussing with his blind friend Italo his plans to marry, his attempts to join the Fascist secret police, and his visits to his parents: a morphine-addicted mother at the family's decaying villa, and his father at an insane asylum.

In a flashback, Marcello is a boy who is humiliated by his schoolmates until he is rescued by Lino, a chauffeur. Lino shows him a pistol and then makes sexual advances towards Marcello. He partially responds to them before grabbing the pistol and shooting into the walls and into Lino. He then flees, believing he committed a murder.

In another flashback, Marcello and Giulia discuss the necessity of his going to confession, even though he is an atheist, in order for her Roman Catholic parents to allow them to marry. Marcello agrees and, in confession, admits to the priest to have committed many sins, including his homosexual intercourse with and subsequent murder of Lino, premarital sex, and his absence of guilt for these sins. Marcello admits he thinks little of Giulia but craves the normality that a traditional marriage with children will bring. The priest is shocked but absolves Marcello once he hears that he is working for the Fascist secret police, called Organization for Vigilance and Repression of Anti-Fascism.

Marcello is then ordered to assassinate his old teacher, Professor Quadri, an outspoken anti-Fascist intellectual now living in exile in France. Using his honeymoon as a cover, he takes Giulia to Paris where he can carry out the mission.

While visiting Quadri, he falls in love with Anna, the professor's wife, and pursues her. Although she and her husband are aware of Marcello's dangerous Fascist sympathies, she responds to his advances and forms a close attachment to Giulia, towards whom she also makes sexual advances. Giulia and Anna dress extravagantly and go to a dance hall with their husbands where Marcello's commitment to the fascists is tested by Quadri. Manganiello is also there, having been following Marcello for some time and doubtful of his intentions. Marcello secretly returns the gun that he has been given and gives Manganiello the location of Quadri's country house where the couple plan to go the following day.

Even though Marcello has warned Anna not to go to the country with her husband, she does make the car journey. On a deserted alpine road, Fascist agents attack and stab Quadri as Anna watches in horror. When the men turn their attention to her, she runs to the car behind for help. When Anna sees that the passenger in the rear of the car is Marcello and realizes his betrayal, she begins to scream, before running into the woods to escape the agents. Marcello watches as she is pursued through the woods and shot to death. Manganiello walks away from the car for a cigarette, disgusted with what he sees as Marcello's cowardice in not shooting Anna when she ran to their car.

In 1943, Marcello now has a child with Giulia and is apparently settled in a conventional life when the resignation of Benito Mussolini and the fascist dictatorship is announced. He is called by Italo for a meeting. While walking, they overhear a conversation between two men; Marcello recognizes one of them as Lino, who survived Marcello's attack. Marcello publicly denounces Lino as a Fascist, homosexual and the murderer of the Quadris. In his frenzy, he also denounces Italo as a Fascist. As a monarchist political crowd sweeps past, taking Italo with them, Marcello sits near a small fire and stares behind him at the man Lino had been talking to, now naked on a bed.