Tired of being rejected by the beautiful women he lusts after, Chuck Barris (Rockwell) moves to Manhattan to become an NBC page with dreams of becoming famous in television but is eventually fired. He moves back to Philadelphia and becomes Dick Clark's personal assistant on American Bandstand in 1961. He writes the successful song "Palisades Park" and becomes romantically involved with a woman named Penny Pacino (Barrymore). Chuck is given permission to pitch the concept for The Dating Game at the American Broadcasting Company (ABC); he receives $7,500 to create a television pilot for the studio. However, ABC abandons The Dating Game in favor of Hootenanny.
One night after Barris is kicked out of a bar for fighting, he is approached by CIA agent Jim Byrd (Clooney), who recruits him as an assassin. Returning from a mission in Mexico, Barris finds that Penny has become a hippie. Meanwhile, ABC decides to greenlight The Dating Game, and by 1967 the TV show is a phenomenon.
Barris takes another mission for the CIA in Helsinki, Finland, where he meets female operative Patricia Watson (Roberts). He finds more success back home when The Newlywed Game goes on air. He and Penny decide to move to Los Angeles into a house, but Barris is cautious of marriage, much to Penny's dismay. The journey in Barris's life is tied in to the story of Thomas Carlyle's main character in Sartor Resartus, Teufelsdrรถckh, and this parallel is referred to throughout the film. In 1970, Byrd convinces Barris to go on another mission in East Berlin to assassinate communist Hans Colbert (Norman Roy). Barris is introduced there to German-American agent Keeler (Rutger Hauer), whom he helps to murder Colbert. However, he is captured by the KGB and, after some weeks, freed during a West-East spies exchange.
In 1976, in Los Angeles, Barris creates The Gong Show and becomes even more famous as its host; he is also criticized for lowering the general quality of television. Meanwhile, Keeler is murdered and Byrd warns Chuck of a mole in the agency. His TV shows are canceled due to poor ratings, and Penny threatens to leave him after catching him cheating on her. One night, Barris finds Byrd sitting atop the diving board of his backyard pool. Byrd reveals to Barris why he was chosen by the CIA to become an assassin: he is the son of a serial killer and has been raised during his infancy as a girl by his mother, so he "fit the profile". Barris threatens to kill Byrd, and the film cuts to a point soon after Byrd is killed, with Barris still pointing his gun at him.
Faced with the unpleasant truth about himself, Barris begins to spiral out of control. After almost having a nervous breakdown on one of his shows, Barris shuts himself away in a New York City hotel. Penny manages to find him there and tries in vain to convince him to return to California to get married.
Barris finally leaves his room and confronts Patricia in Boston. After a cup of coffee with her, Barris falls to the floor, seemingly poisoned. Patricia then reveals that she is the mole. However, Barris actually tricked Patricia into drinking from the poisoned cup, and he himself wasn't actually poisoned at all. After her death, he returns home and begins to write his autobiography, Confessions of a Dangerous Mind. He finally decides to marry Penny. At the end of the ceremony, he notices some of the people he previously killed among the crowd. Distraught, he confesses to her his double life as a CIA agent and assassin, but she merely laughs, assuming he is joking, and he decides not to correct her. In 2002, he prepares for an interview for the film adaptation of his autobiography.

Question: What is the truth that Byrd reveals to Chuck?
Answer: That is father was a serial killer.
Alternative answer: he's the son of a serial killer