Citizen Kane opens with the camera panning across a spooky, seemingly deserted estate in Florida called Xanadu. The camera lingers on a "No Trespassing" sign and a large "K" wrought on the gate, then gradually makes its way to the house, where it appears to pass through a lit window. A person is lying on a slab-like bed. Snowflakes suddenly fill the screen. As the camera pulls back, a snow-covered cabin comes into view. The camera pulls back more quickly to show that what we have been looking at is actually just a scene inside a snow globe in the hand of an old man. The camera focuses on the old man’s mouth, which whispers one word: "Rosebud." He then drops the globe, which rolls onto the floor and shatters. Reflected in the curve of a piece of shattered glass, a door opens and a white-uniformed nurse comes into the room. She folds the old man’s arms over his chest and covers his face with a sheet.

In the next scene, a newsreel entitled News on the March announces the death of Charles Foster Kane, a famous, once-influential newspaper publisher. The newsreel, which acts as a lengthy obituary, gives an overview of Kane’s colorful life and career and introduces some of the important people and events in Kane’s life. The newsreel plays in a small projection room filled with reporters. The producer of the newsreel tells the reporters he’s not happy with the film because it merely recounts Kane’s life, instead of revealing who Kane truly was. He notes that Kane’s last word was "Rosebud" and wonders if that may hold the key to Kane’s character. He decides to stall the newsreel’s release and sends a reporter, Jerry Thompson, to talk to Kane’s former associates to try to uncover the identity of Rosebud.

Thompson first interviews Kane’s ex-wife, Susan Alexander Kane, who works as a dancer and singer in a dingy bar. Susan is drunk and uncooperative. A waiter hovers over her and tells Thompson that Susan has been unwilling to talk about Kane since he died, although she spoke of him often when he was alive. The waiter also says he asked Susan about Rosebud after Kane died and she claimed she’d never heard of Rosebud. Thompson then goes to the bank that houses the memoirs of Kane’s childhood guardian, Walter Parks Thatcher. As Thompson begins to read these memoirs, the image of the page dissolves into a flashback to Kane’s childhood.

A roughly chronological series of flashbacks tells Kane’s life story from five different points of view. The first flashback shows how Thatcher meets Kane. Kane’s mother, Mary, runs a boarding house in rural Colorado. In lieu of a payment, one of her tenants gives her some stock in what she thinks is a worthless mine; it turns out to give her ownership of the Colorado Lode, a working gold mine. Finding herself suddenly wealthy, she decides to send away her son, Charles, to be raised by her banker, Thatcher. Charles is understandably upset and whacks Thatcher with the sled he's been happily riding when Thatcher shows up to escort him away. Kane’s relationship with Thatcher never improves. Vignettes from their years together show Kane engaging in questionable journalism, wasting money, and constantly enraging Thatcher.

Thompson interviews other people who were close to Kane, and these characters relate their memories of the man through flashbacks as well. Thompson speaks first with Kane’s good friends and employees, Mr. Bernstein and Jedediah Leland, and has one more conversation with his ex-wife Susan. Most significantly, Thompson interviews the butler, Raymond, who remembers Kane saying “Rosebud” following a violent episode after Susan left him. Each person gives his or her own version of an abandoned, lonely boy who grows up to be an isolated, needy man. All reveal in some way that Kane is arrogant, thoughtless, morally bankrupt, desperate for attention, and incapable of giving love. These faults eventually cause Kane to lose his paper, fortune, friends, and beloved second wife, Susan. Thompson, the reporter, never does find out what Kane meant by "Rosebud." Giving up the quest, Thompson is leaving Kane’s abandoned castle, Xanadu, when the camera pans a scene of workers burning some of Kane’s less valuable possessions. In the fire is the sled that Kane was riding the day his mother sent him away. Painted on the sled is the name Rosebud.