New York, 1945. In a dim office, while his daughter Connie's wedding reception roars outside, DON VITO CORLEONE receives supplicants. The undertaker BONASERA begs for vengeance against the men who beat his daughter. The Don chastises him for never seeking friendship before, then grants the favor — on the condition that Bonasera may one day be called upon to repay it. The baker NAZORINE asks that his daughter's Italian boyfriend be allowed to stay in America. Each request is handled with quiet authority, delegated through TOM HAGEN, the Don's adopted son and consigliere.
Outside, the wedding is a sprawling Sicilian-American celebration. SONNY CORLEONE, the Don's hot-tempered eldest, sneaks upstairs with his mistress LUCY MANCINI while his wife SANDRA jokes about his anatomy with the other women. FREDO, the gentle middle son, mingles awkwardly. FBI agents photograph license plates at the gate, and Sonny smashes a photographer's camera. The fearsome enforcer LUCA BRASI rehearses a simple thank-you speech, stumbling over his words when he finally delivers it to the Don.
MICHAEL CORLEONE arrives late with his girlfriend KAY ADAMS, a New England schoolteacher. He's in his Marine uniform, visibly separate from this world. Over dinner, he tells Kay the story of how his father helped his godson JOHNNY FONTANE by having Luca Brasi hold a gun to a bandleader's head. Michael delivers the punchline — 'My father made him an offer he couldn't refuse' — then adds quietly: 'That's my family, Kay. It's not me.'
Johnny Fontane arrives from Hollywood and privately begs the Don for help landing a career-saving movie role. The Don slaps him for whining, then promises to handle it. He dispatches Tom to Los Angeles to negotiate with studio head JACK WOLTZ. Woltz refuses violently, ranting about Johnny ruining his protégée. Tom leaves politely. The next morning, Woltz wakes in a pool of blood to find the severed head of his prized $600,000 racehorse in his bed.
Back in New York, the Don considers a proposition from VIRGIL SOLLOZZO, a drug trafficker backed by the Tattaglia family. Sollozzo wants Corleone money, political connections, and legal protection for his narcotics operation. Tom advises yes — drugs are the future. But the Don refuses, seeing narcotics as a business that would cost him his political allies. During the meeting, Sonny blurts out interest in the deal, a mistake the Don furiously reprimands him for. Sollozzo notices Sonny's enthusiasm.
The Don sends Luca Brasi to infiltrate the Tattaglias and learn Sollozzo's plans. At a bar meeting, Sollozzo and Bruno Tattaglia murder Luca — his hand pinned to the bar with a knife, garroted from behind. On a city street, Sollozzo's men kidnap Tom Hagen. And outside his olive oil office on a winter evening, the Don is gunned down while buying fruit. Five bullets. Fredo, left to guard him, sits on the curb weeping, unable to even hold his gun.
Michael and Kay learn the news from a newspaper headline outside Radio City Music Hall. Michael calls Sonny, who tells him to come home. At the Corleone compound, Sonny takes command. He orders Clemenza to kill PAULIE GATTO, the bodyguard who called in sick the day of the shooting — clearly the inside man. Clemenza executes Paulie on a quiet road after stopping to relieve himself. 'Leave the gun. Take the cannoli.'
Sollozzo releases Tom with a message: the Don is dead, make peace. But the Don survives. Sollozzo, desperate, arranges through Captain McCLUSKEY — a corrupt police captain on his payroll — to have the hospital guards removed. Michael arrives at the hospital to find his father completely unprotected. He moves the Don to another room with help from a nurse and ENZO THE BAKER, who happened to arrive with flowers. Outside, Michael and Enzo bluff away Sollozzo's hitmen by standing at the entrance pretending to be armed. Michael notices his hands aren't shaking. McCluskey arrives, punches Michael in the jaw, and is only stopped from arresting him when Tom appears with Corleone lawyers.
At the compound, Sonny wants all-out war. Tom counsels patience. Then Michael — the civilian, the college boy, the war hero who wanted nothing to do with the business — speaks up. He proposes killing both Sollozzo and McCluskey at a restaurant meeting. The room laughs. Sonny mocks him. But Michael lays out the plan with cold precision: a planted gun, a public place, two shots apiece in the head. 'It's not personal, Sonny. It's strictly business.'
Clemenza coaches Michael on the mechanics — how to hold the gun, how to walk out, how to disappear. At Louis' Restaurant in the Bronx, Michael sits across from Sollozzo and McCluskey. Sollozzo speaks in Italian about peace. McCluskey eats his veal. Michael excuses himself to the bathroom, finds the gun taped behind the toilet tank, and returns. A train roars past. Michael shoots Sollozzo in the forehead, then McCluskey in the throat and head. He drops the gun and walks out. Tessio picks him up.
Michael is smuggled to Sicily, where he lives under the protection of DON TOMMASINO. He walks the countryside with bodyguards FABRIZIO and CALO, visits the village of Corleone where all the men are dead from vendettas, and falls thunderstruck for APOLLONIA, a village girl. He courts her formally, meets her father VITELLI, and marries her in a traditional Sicilian ceremony.
Back in New York, the war grinds on. Newspaper montages track the violence. Sonny discovers Carlo has beaten Connie and nearly kills him in the street, biting his fist and slamming a garbage can lid on his head. The beating of Connie becomes a weapon: Carlo provokes another fight, Connie calls Sonny in hysterics, and Sonny races out alone. At the causeway tollbooth, he's ambushed — riddled with bullets from a line of gunmen, then shot again and kicked after he falls.
The Don, still recovering, absorbs the news from Tom. He calls for a meeting of the Five Families. He calls Bonasera to prepare Sonny's body so his mother won't see what they did to him. 'Look how they massacred my boy.'
In Sicily, Michael learns of Sonny's death. Tommasino warns him to move to a safer location. As Michael prepares to leave, Apollonia insists on driving. Fabrizio — revealed as a traitor — walks quickly through the gate. Michael sees him and screams for Apollonia to stop. The car explodes.
At the meeting of the Five Families, the Don brokers peace. He agrees to share his political connections and allow the drug trade, provided it's kept away from schools and children. He forgoes vengeance for Sonny. His only condition: Michael must be allowed to return safely. He warns the assembled Dons that if anything happens to Michael, he will hold them responsible. In the car afterward, Tom mentions Tattaglia. The Don corrects him: 'Tattaglia's a pimp. He never could've outfought Santino. But I didn't know until this day that it was Barzini all along.'
Michael returns to America. He finds Kay teaching school in New Hampshire. He tells her he's working for his father now. She challenges him — senators and presidents don't have men killed. 'Oh, who's being naive, Kay?' He promises the family will be legitimate in five years. He asks her to marry him. She resists, then relents.
The Don, aging and diminished, counsels Michael in the garden. He warns that Barzini will move against him, and that whoever arranges the meeting will be the traitor. He confesses he never wanted this life for Michael — he'd imagined Senator Corleone, Governor Corleone. Michael reassures him. In a later scene, the Don plays with his grandson Anthony in the tomato garden, puts an orange peel over his teeth to make a monster face, chases the boy through the plants, and collapses. He dies among the tomatoes.
At the funeral, Tessio approaches Michael to arrange a meeting with Barzini — revealing himself as the traitor the Don predicted. Michael agrees, then tells Tom: he'll wait until after the baptism.
The baptism of Connie and Carlo's son becomes the film's devastating climax. As Michael stands at the altar renouncing Satan, his assassins fan out across the city. Clemenza guns down Strachi in an elevator. A hitman shoots Moe Greene through his eyeglasses. Cicci traps Don Cuneo in a revolving door and fires through the glass. Rocco and another gunman riddle Tattaglia and a woman in a motel bed. Neri, dressed in his old police uniform, shoots Barzini on the courthouse steps. 'Michael Rizzi, do you renounce Satan?' 'I do renounce him.'
Afterward, Michael confronts Carlo. He knows Carlo set up Sonny — the staged beating of Connie was the bait. Carlo confesses it was Barzini who approached him. Michael promises Carlo he's merely out of the family business, hands him a plane ticket to Vegas. Carlo gets into a car. Clemenza is in the back seat. He garrotes Carlo as the car pulls away.
Connie storms into Michael's office, screaming that he killed her husband, that he waited until their father died so no one could stop him. Kay watches, shaken. After Connie is taken upstairs, Kay asks Michael directly: is it true? He tells her never to ask about his business. She insists. He allows it — this one time. 'Is it true?' He shakes his head. 'No.' Kay exhales with relief. She leaves to fix drinks. As she turns back, she sees Clemenza, Rocco, and Neri entering the office. They kiss Michael's hand. 'Don Corleone.' Neri closes the door, and Kay is shut out.