In 1945, during the wedding of his daughter, Connie, Vito Corleone listens to requests in his role as the Godfather, Don of a Crime Family in New York. Michael, Vito’s youngest son who served as a Marine during the Second World War, introduces Kay Adams, his girlfriend, to his family at the reception. Also, Johnny Fontaine, Vito’s godson and a popular singer, requests for Vito’s assistance in helping him get a movie role.
Vito goes on to send Tom Hagen, his consigliere, to Los Angeles to persuade the over confident studio head Jack Woltz to give the role to Johnny. Woltz refuses to comply and wakes up in bed to meet the severed head of his favorite horse. Before Christmas, Virgil “The Turk” Sollozo, a drug baron under the protection of the Tattagalia crime family, requests for Vito to invest in his narcotics business and also help protect it with his political connections.
Vito turns down his offer as he does not want to be associated in a dangerous trade that risks alienating political insiders. Vito grows suspicious of his plans and sends Luca Brasi, his enforcer, to secretly spy on them. However, Brasi is killed by a Tattagalia button man at his first meeting with Bruno Tattagalia and Sollozo.
Much later, Sollozo orders for Vito to be shot in the street and abducts Hagen. This puts Sonny Vito’s first-born, in charge of activities. Sollozo pressures Hagen to convince Sonny to accept his deal and then later sets him free. The family is then sent a fish that is wrapped in Brasi’s bullet proof vest, which is interpreted to mean that “Luca sleeps with the fishes.”
Meanwhile, Vito recovers at the hospital and another attempt is made on his life but Michael is able to stop the assailants. Michael gets his jaw broken by Sollozo’s Bodyguard, NYPD Captain Marc McCluskey and Sonny retaliates by hitting Bruno Tattagalia. Michael plans to assassinate Sollozo and McCluskey on the pretext of dispute resolution; he kills them both, with a planted handgun, after agreeing to meet them in a Bronx restaurant.
Regardless of the intervention of the authorities, the five crime families begin an open street warfare, causing Vito’s sons to fear for their lives. Michael hides out in Sicily and Credo takes shelter with Carlo Greene in Las Vegas. Later, Sonny ambushes Carlo Greene, his brother-in-law, on the street for abusing his sister and promises to have him killed if it ever happened again. When Sonny’s sister gets abused again, he rushes to their home but gets attacked at a highway toll booth, where he is killed with machine guns.
Meanwhile, in Sicily, Michael meets and marries Apollonian Vitelli but she is killed by a bomb that was meant for him. Vito is left devastated by Sonny’s death and begins to make moves to end the war. He discovers that the Tattagalia are now under the leadership of Don Emilio Barzini. Vito tells the Five Families that he will draw back his forces from their heroin business and let go of avenging his son’s murder.
Vito’s promise guarantees Michael’s safety and allows him to come back home and be a part of the family business. He gets married to Kay and promises her that the family business will be legal in five years. Kay births two children for him in early 1950. As Vito draws closer to the end of his career, Michael takes over the family’s activities. He convinces Hagen to relocate to Las Vegas and relinquish his position to Vito because Tom is not a “Wartime Consigliere.” Vito agrees to this idea and remarks that Tom should “have no part in what will happen” in the future battle with the five families.
Michael heads off to Las Vegas to purchase Greene’s stakes in the family’s casinos and is shocked to see that Credo is now under the influence of Greene. In 1955, Vito dies from a severe heart attack and at his burial, a Corleone capo named Tessio asks Michael to meet with Don’t Barzini, confirming the betrayal that Vito had warned about.
The meeting takes place on the same day as Connie’s baby’s christening. When the day comes, Michael takes his place at the altar as the baby’s godfather, and assassins of the Corleone family murder the other Dons of New York along with Moe Greene. Tessio is killed for his betrayal and Michael gets Carlo’s confession on how he played a major role in helping Barzini murder Sunny. Right after this, Clemenza; another Corleone capo, garrotes Carlo with a wire.
Connie goes onto accuse Michael of the murder and informs Kay that Michael was the one who ordered all the murders. Kay however remains calm when Michael denies the accusations. The capos eventually arrive and start addressing Michael as Don Corleon, and she observes as the all walk away, closing the door behind them.