In late summer 1945, guests are gathered for the wedding reception of Don Vito Corleone's daughter Connie (Talia Shire) and Carlo Rizzi (Gianni Russo). Vito (Marlon Brando), the head of the Corleone Mafia family who is known to friends and associates as "Godfather" and Tom Hagen (Robert Duvall), the Corleone family lawyer and consigliere (counselor), are hearing requests for favors because "no Sicilian can refuse a request on his daughter's wedding day". Meanwhile, the Don's youngest son Michael (Al Pacino), a decorated Marine war hero returning from World War II service, tells his girlfriend Kay Adams (Diane Keaton) anecdotes about his family, attempting to inform her about his father's criminal life; he reassures her that he is different from his family. Among depicting the marriage of Connie and Carlo, the wedding scene also serves as a critical exposition scene for the remainder of the film, as Michael figuratively introduces the main characters to Kay.

Among the guests at the celebration is the famous singer Johnny Fontane (Al Martino), Corleone's godson, who has come from Hollywood to petition Vito's help in landing a movie role that will revitalize his flagging career. Jack Woltz (John Marley), the head of the studio, denies Fontane the part, but Don Corleone explains to Johnny: "I'm gonna make him an offer he can't refuse." Hagen is dispatched to California to fix the problem, but Woltz angrily tells him that he will never cast Fontane in the role, for which he is perfect and will make him an even bigger star, because Fontane seduced and "ruined" a starlet that Woltz favored. Woltz is persuaded otherwise, however, when he wakes up early the next morning and feels something wet in his bed. He pulls back the sheets, and finds himself in a pool of blood with the severed head of his prized $600,000 stud horse, Khartoum, in the bed with him, and screams in horror. (A deleted scene from the film implies that Luca Brasi, Vito's top "button man" or hitman, is responsible.)

Upon Hagen's return, the family meets with Virgil "The Turk" Sollozzo (Al Lettieri), who is being backed by the rival Tattaglia family. He asks Don Corleone for financing, and political and legal protection for importing and distributing heroin. Despite the huge profit to be made, Corleone refuses, explaining that his political influence would be jeopardized by a move into the narcotics trade. The Don's eldest son, hotheaded Sonny (James Caan), who had earlier urged the family to enter into the narcotics trade, breaks ranks during the meeting and questions Sollozzo's assurances as to the Corleone Family's investment being guaranteed by the Tattaglia Family. His father, angry at Sonny's dissension in a non-family member's presence, privately rebukes him later. Don Corleone then dispatches Luca Brasi (Lenny Montana) to infiltrate Sollozzo's organization and report back with information. During the meeting, whilst Brasi is bent over to allow Bruno Tattaglia to light his ciga