The film intercuts between events some time after The Godfather and the early life of Vito Corleone.
Vito
In 1901, the family of nine-year-old Vito Andolini is killed in Corleone, Sicily, after his father insults local Mafia chieftain Don Ciccio. Vito escapes on a ship to New York City, and is registered as "Vito Corleone" on Ellis Island. He settles in New York City and in 1917 has a son Sonny with his wife. He loses his job due to the interference of a local don, Fanucci; Clemenza invites Vito to take part in a burglary unwittingly.
Vito has two more sons, Fredo and Michael. Vito's criminal conduct attracts the attention of Fanucci, who extorts him. Vito's partners, Clemenza and Salvatore Tessio, suggest paying him, but Vito insists Fanucci will accept a smaller payment if they make him "an offer he can't refuse". During a neighborhood festa, he stalks Fanucci to his apartment and shoots him dead. Vito becomes a respected and successful member of the community and is approached for help by a widow who is being evicted. After the widow's landlord learns of Vito's reputation, he agrees to let the widow stay on very favorable terms. Vito, along with his family, visits Sicily for the first time since emigrating. He and business partner Tommasino are admitted to Don Ciccio's compound, ostensibly to ask for Ciccio's blessing on their olive oil business. Vito exacts vengeance, killing Ciccio after revealing his former identity.
In the 1940s, the Corleone family gathers in their dining room to surprise Vito for his birthday. Michael announces that, in response to the attack on Pearl Harbor, he has left college and enlisted in the United States Marine Corps, leaving Sonny furious, Tom incredulous, and Fredo the only supportive brother. When Vito is heard at the door, all but Michael leave the room to greet him.
Michael
In 1958, during his son's First Communion party at Lake Tahoe, Michael has a series of meetings in his role as the don of the Corleone crime family. Frank Pentangeli, a Corleone capo, is dismayed that Michael refuses to help defend his territory against the Rosato brothers, who work for Hyman Roth, a long-standing Corleone business partner. That night, Michael leaves Nevada after surviving an assassination attempt at his home.
Michael suspects Roth planned the assassination, but feigns ignorance when they meet. In New York City, Pentangeli attempts to maintain Michael's pretense by making peace with the Rosato family but they try to kill him. Roth, Michael, and several of their partners travel to Havana to discuss their future Cuban business prospects under the cooperative government of Fulgencio Batista. Michael becomes reluctant after reconsidering the viability of the ongoing Cuban Revolution. On New Year's Eve, Fredo inadvertently reveals that he knows Roth's right-hand man, Johnny Ola, having previously claimed that they had never met, and Michael realizes that it was Fredo who had betrayed him. Michael orders his bodyguard kill Ola and Roth, but the bodyguard is killed by the police before he can reach Roth. Batista abdicates due to rebel advances. During the ensuing chaos, Michael, Fredo, and Roth separately escape to the United States. Back home, Michael learns that his wife Kay has miscarried.
In Washington, D.C., a Senate committee on organized crime is investigating the Corleone family. Pentangeli agrees to testify against Michael, who he believes had double-crossed him, and is placed under witness protection. On returning to Nevada, Fredo tells Michael that he felt resentful for being disregarded. He claims to be ignorant of the plot on Michael's life and informs Michael that the Senate lawyer is on Roth's payroll. Michael disowns Fredo but orders a capo to continue protecting him while their mother is alive. Michael sends for Pentangeli's brother from Sicily, resulting in Pentangeli renouncing his previous statement; the hearing dissolves in an uproar. Kay reveals to Michael that she actually had an abortion, not a miscarriage and that she intends to remove their children from Michael's criminal life. Outraged, Michael strikes Kay, banishes her from the family, and takes custody of the children.
Michael's mother dies and Michael moves to wrap up loose ends. Roth is forced to return to the United States after being refused asylum and entry to Israel. Michael orders capo Rocco Lampone to assassinate Roth, who shoots Roth dead at Miami International Airport. At the witness protection compound, consigliere Tom Hagen reminds Pentangeli that failed plotters against the Roman Emperor often committed suicide and assures him that his family will be cared for. Pentangeli slits his wrists in his bathtub. Al Neri, acting on Michael's orders, kills Fredo.
Al Pacino as Michael
Robert Duvall as Tom Hagen
Diane Keaton as Kay
Robert De Niro as Vito Corleone
John Cazale as Fredo Corleone
Talia Shire as Connie Corleone
Lee Strasberg as Hyman Roth
Michael V. Gazzo as Frank Pentangeli
G. D. Spradlin as Senator Pat Geary
Richard Bright as Al Neri
Gastone Moschin as Fanucci
Tom Rosqui as Rocco Lampone
B. Kirby, Jr. as young Clemenza
Frank Sivero as Genco
Francesca De Sapio as young Mama Corleone
Morgana King as Mama Corleone
Mariana Hill as Deanna Corleone
Leopoldo Trieste as Signor Roberto
Dominic Chianese as Johnny Ola
Amerigo Tot as Michael's bodyguard
Troy Donahue as Merle Johnson
John Aprea as young Tessio
Joe Spinell as Willi Cicci