In 1945 New York City, at his daughter Connie’s wedding to Carlo, Vito Corleone listens to requests in his role as don of the Corleone crime family. 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 has Vito gunned down in the street, then kidnaps Hagen. 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. Michael plots to murder Sollozzo and McCluskey; feigning a desire to settle the dispute, Michael meets them in a Bronx restaurant, where after retrieving a handgun planted by Clemenza, a Corleone capo, he kills both men.
Despite a clampdown by the authorities, the Five Families erupt in open warfare, and Vito fears for his sons’ safety. Michael takes refuge in Sicily and 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. He insists Hagen relocate to Las Vegas and 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, Vito suffers a fatal heart attack. At the funeral, Tessio, a Corleone capo, asks Michael to meet with Barzini, signaling the betrayal that Vito had forewarned. The meeting is 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 the killings. Kay is relieved when Michael finally denies it, but when the capos arrive, they address her husband as Don Corleone and she watches them pay reverence to Michael as the newly installed don as they close the door on her.