Antiques dealer Alfredo returns to his exclusive apartment in Rome in the early morning hours. When his fiancée Nicoletta rings him up on the phone, he pretends that he had spent the whole night at home. Shortly after, the police arrive and arrest Alfredo, as his former mistress Adalgisa, with whom he had spent the previous night, has been found murdered in her hotel at the seaside. Also, Nicoletta has suddenly disappeared. While Alfredo is in custody, the police research his past and uncover an egotistical, ruthless careerist who uses the people around him to climb the social ladder at any cost, selling fake antiques, having his mistress pay for his expenses and dropping her at the prospect of marrying Nicoletta, the daughter of a rich industrialist. When Nicoletta shows up again and the true murderer is convicted, Alfredo is released. Showing remorse for his past behaviour at first, Alfredo soon returns to his previous ways.
The Assassin was released in Italy on 1 April 1961, distributed by Titanus.[2] It was shown in competition at the 11th Berlin International Film Festival June–July the same year and released in France in July 1964.[2]