In 1945 New York City, at his daughter Connie's wedding to Carlo, Vito Corleone, the don of the Corleone crime family listens to requests. His youngest son, Michael, who was a Marine during World War II, introduces his girlfriend, Kay Adams, to his family at the reception. Johnny Fontane, a popular singer and Vito's godson, seeks Vito's help in securing a movie role; Vito dispatches his consigliere, Tom Hagen, to Los Angeles to persuade studio head Jack Woltz to give Johnny the part. Woltz refuses until he wakes up in bed with the severed head of his prized stallion.
Shortly before Christmas, drug baron Sollozzo, backed by the Tattaglia crime family, asks Vito for investment in his narcotics business and protection through his political connections. Wary of involvement in a dangerous new trade that risks alienating political insiders, Vito declines. Suspicious, Vito sends his enforcer, Luca Brasi, to spy on them. Brasi is garroted to death during his first meeting with Bruno Tattaglia and Sollozzo. Later, Sollozzo kidnaps Hagen, then has Vito gunned down in the street. With Corleone first-born Sonny in command, Sollozzo pressures Hagen to persuade Sonny to accept Sollozzo's deal, then releases him. The family receives fish wrapped in Brasi's bullet-proof vest, indicating that Luca "sleeps with the fishes". Vito survives, and at the hospital, Michael thwarts another attempt on his father. Michael's jaw is broken by NYPD Capt. McCluskey, Sollozzo's unofficial bodyguard. Sonny retaliates with a hit on Tattaglia's son. Michael plots to murder Sollozzo and McCluskey; feigning a desire to settle the dispute, Michael meets them in a Bronx restaurant in which, after retrieving a handgun planted by Peter Clemenza, a Corleone capo, he kills both men.
Despite a clampdown by the authorities, war breaks out between the Five Families and Vito fears for his family. Michael takes refuge in Sicily and his elder brother Fredo is sheltered by Moe Greene in Las Vegas. Sonny attacks Carlo on the street for abusing Connie and threatens to kill him if it happens again. When it does, Sonny speeds to their home but is ambushed at a highway toll booth and violently murdered by gangsters wielding submachine guns. While in Sicily, Michael meets and marries Apollonia, but a car bomb intended for him takes her life.
Devastated by Sonny's death and realizing that the Tattaglias are controlled by the now-dominant don, Barzini, Vito attempts to end the feud. He assures the Five Families that he will withdraw his opposition to their heroin business and forgo avenging Sonny's murder. His safety guaranteed, Michael returns home to enter the family business and marry Kay, promising her that the business will be legitimate within five years. Kay gives birth to two children by the early 1950s. With his father nearing the end of his life and Fredo too weak, Michael takes the family reins, starting to move the family's business to Las Vegas. Expecting trouble due to this move, he insists Hagen also relocate to Las Vegas but relinquish his role to Vito because Hagen is not a "wartime consigliere". Vito agrees Hagen should "have no part in what will happen" in the coming battles with the rival families. When Michael travels to Las Vegas to buy out Greene's stake in the family's casinos, he is dismayed to see that Fredo is more loyal to Greene than to his own family.
In 1955, after warning Michael that whoever agrees to arrange a meeting between him and Barzini is a traitor, Vito suffers a fatal heart attack. At the funeral, Tessio, a Corleone capo, asks Michael to meet with Barzini, with the meeting set for the same day as the baptism of Connie's baby. While Michael stands at the altar as the child's godfather, Corleone hitmen murder the other New York City dons and Greene. Tessio's treachery leads to his execution. Michael extracts Carlo's confession to his complicity in setting up Sonny's murder for Barzini; Clemenza garrotes Carlo to death. Connie accuses Michael of the murder, telling Kay that Michael ordered all of the killings. Kay is at first relieved when Michael denies it, but later dismayed when she watches the capos arrive to pay reverence to her husband as Don Corleone and their closing the door on her.
Marlon Brando as Vito Corleone: crime boss and patriarch of the Corleone family
Al Pacino as Michael: Vito's youngest son
James Caan as Sonny: Vito's eldest son
Richard Castellano as Clemenza: a caporegime in the Corleone crime family, Sonny's godfather
Robert Duvall as Tom Hagen: Corleone consigliere, lawyer, and unofficial adopted member of the Corleone family
Sterling Hayden as Capt. McCluskey: a corrupt police captain on Sollozzo's payroll
John Marley as Jack Woltz: Hollywood film producer who is intimidated by the Corleones
Richard Conte as Barzini: a crime boss of a rival crime family
Al Lettieri as Sollozzo: an adversary who pressures Vito to get into the drug business, backed by the Tattaglia family
Diane Keaton as Kay Adams: Michael's girlfriend and, later, second wife
Abe Vigoda as Tessio: a caporegime in the Corleone crime family
Talia Shire as Connie: Vito's only daughter
Gianni Russo as Carlo: Connie's abusive husband
John Cazale as Fredo: Vito's second son
Rudy Bond as Cuneo: a crime boss of a rival crime family
Al Martino as Johnny Fontane: singer
Morgana King as Mama Corleone: Vito's wife
Lenny Montana as Luca Brasi: Vito's enforcer
Johnny Martino as Paulie Gatto: a soldier in the Corleone crime family
Salvatore Corsitto as Bonasera: the undertaker who asks for a favor at Connie's wedding
Richard Bright as Neri: the soldier in the Corleone crime family who becomes Michael's enforcer
Other actors playing smaller roles are Alex Rocco as Moe Greene, Simonetta Stefanelli as Apollonia Vitelli-Corleone, Tony Giorgio as Bruno Tattaglia, Angelo Infanti as Fabrizio, Franco Citti as Calò, Joe Spinell as Willi Cicci and Corrado Gaipa as Don Tommasino