Seventy-year-old newspaper tycoon Charles Foster Kane dies in his palatial Florida home, Xanadu, after uttering the single word “Rosebud.” While watching a newsreel summarizing the years during which Kane built a dying newspaper into a major empire, married and divorced twice, ran unsuccessfully for governor and saw the collapse of his newspaper empire during the Depression, an editor decides they have not captured the essence of the controversial newspaperman and assigns reporter Jerry Thompson to discover the meaning of Kane's last word.
Thompson first approaches Kane's second wife, singer Susan Alexander, in the Atlantic City nightclub where she now performs. After the drunken Susan orders Thompson to leave, the accommodating bartender reports her claim that she had never heard of Rosebud. Next, Thompson reads the unpublished memoirs of Wall Street financier Walter Parks Thatcher, Kane's guardian and trustee of the mining fortune left to Kane by his mother: Thatcher first meets young Kane in 1871 at his mother's Colorado boardinghouse. Learning that she has become wealthy from mining shares left her by a former boarder, she is determined that her son will be reared and educated in the East. As young Charlie plays outside with his sled, Mrs. Kane hands over management of the mine's returns to Thatcher, against her husband's wishes, then grants the financier guardianship over her son. Despite the boy's protests, he is sent away to live with Thatcher. When Kane turns twenty-five, he assumes control of the world's sixth largest private fortune, and while professing disinterest in most of his holdings, writes Thatcher that he intends to run The Inquirer , a small, New York newspaper acquired through a foreclosure. He moves into the paper's offices and with the help of his best friend, Jedidiah Leland, who acts as the drama critic, turns it into a lively, muckraking publication, which attacks slum landlords, swindlers and big business. In 1898, The Inquirer attempts to draw the United States into war with Spain. After the 1929 stock market crash, Kane relinquishes control of his empire to Thatcher's syndicate. Thompson finishes his reading of Thatcher's memoir without learning anything about Rosebud.
Thompson next questions Bernstein, formerly Kane's general editor and now chairman of the board. Bernstein describes the early days of Kane's tenure at The Inquirer : After Kane and Leland take over the publication in 1892, Kane prints a declaration of principles--that he will report the news honestly and will make the paper a champion of his readers' rights as citizens and as human beings. Leland senses the document's importance and keeps the handwritten declaration as a memorial. Six years later, when Kane acquires the top reporters from the rival paper, whose circulation The Inquirer has surpassed, Leland worries that Kane's approach to the news will also resemble his rival's. During this period, Kane begins to collect the European statues and furniture that will later crowd the rooms of Xanadu. On one European trip, Kane meets and becomes engaged to Emily Monroe Norton, the President's niece, whom he marries in 1900. After relating these events, Bernstein suggests that Rosebud was probably something that Kane lost, perhaps a woman.
Taking Bernstein's advice, Thompson visits Leland, a self-described “disagreeable old man,” in the hospital where he is living out his old age. Leland claims Kane believed in nothing except himself, but suggests that Kane's story is about how he lost love because he had none to give: As Kane's empire expands, his marriage to Emily deteriorates. One night in 1915, Kane encounters Susan as she is leaving a pharmacy after purchasing a toothache remedy. Susan innocently offers to let Kane, who has been spattered by mud from a passing carriage, use her apartment to clean up. Kane is at ease with Susan, who has no idea of his importance, and when he learns that her mother wanted her to become an opera singer, requests that she sing for him. In 1916, Kane runs for governor against corrupt political boss Jim Gettys. After a successful campaign speech, Emily sends their son home alone and asks Kane to accompany her to Susan's boardinghouse, where they find Gettys with Susan. Gettys admits that he forced Susan to contact Emily and tells Kane that he will reveal their relationship unless he withdraws from the campaign. Despite the hurt that scandal will bring to his family and Susan, Kane refuses, convinced that he has the love of the electorate. He is mistaken, however, and loses the race. Leland accuses Kane of treating “the people” as if he owned them and asks to be transferred to The Inquirer 's Chicago branch. After Emily divorces him, Kane marries Susan and in 1919, builds the Chicago Opera House for her. Susan's voice is very poor, however, and her debut is met with ridicule, except by The Inquirer critics. When Kane finds Leland slumped over his typewriter in a drunken stupor after beginning an unfavorable review of Susan's performance, he finishes the notice himself, retaining the negative viewpoint, but then fires his old friend.
Thompson now returns to Atlantic City to question Susan again. She insists that it was Kane's idea that she have an operatic career and describes their tempestuous life together: During a noisy quarrel with Susan, Kane receives a special delivery from Leland, returning the $25,000 check Kane sent after firing him and including the handwritten copy of the declaration of principles, which Kane burns. When Susan begs to quit, Kane insists that he will be humiliated if she leaves the stage, and forces her to continue singing until she attempts suicide. Later, they retire to Xanadu, where a bored Susan spends her days working jigsaw puzzles. Finally fed up with his overbearing attempts to orchestrate her life, Susan reproaches Kane for trying to buy her affections with jewels and other material things. He slaps her in anger, and she leaves him. Her story finished, Susan sends Thompson to talk to Raymond, the butler at Xanadu. Thompson confesses to Susan that he feels sorry for Kane, and Susan admits that she does, too.
At Xanadu, Raymond agrees to speak with Thompson for a price, then relates the events following Susan's departure: The furious Kane tears apart Susan's room, until he comes across a small glass snow globe with a tiny cabin inside. Kane picks it up, murmurs “Rosebud” and leaves the room, seemingly unaware of the servants who surround him. Still as ignorant of the significance of Kane's dying word as when he started, Thompson prepares to leave Xanadu with the other reporters and photographers. Passing through rooms where Kane's possessions are being inventoried and crated, Thompson is now convinced that even if he had learned the meaning of Rosebud, it would not have explained the man. Unnoticed among the boxes and crates is an old child's sled. As a workman throws the sled into a furnace, the word Rosebud, painted across the top, is consumed by the flames.