Broadway star Margo Channing recently turned 40 and worries about what advancing age will mean for her career. After a performance of Margo's latest play, Margo's close friend, Karen Richards, wife of the play's author Lloyd Richards, brings besotted fan Eve Harrington backstage to meet Margo. In Margo's dressing room, Eve tells Karen, Lloyd, and Margo's maid Birdie, that she followed Margo's last theatrical tour to New York City after seeing her perform in San Francisco. She tells an engrossing story of growing up poor in Wisconsin and losing her young husband, Eddie in the South Pacific during World War II. Margo, moved by Eve's story, takes her into her home as her assistant, upsetting Birdie.
Eve quickly manipulates her way into Margo's life, acting as both secretary and adoring fan. She places a long-distance phone call to Margo's boyfriend Bill Sampson when Margo forgets his birthday. Margo grows increasingly distrustful and bitter toward Eve, particularly after catching Eve taking a bow to an empty theater while pretending to wear Margo's costume. Margo asks producer Max Fabian to hire Eve at his office, but instead, Eve becomes Margo's understudy without Margo's knowledge.
The film's trailerAs Margo's irritation grows, Karen sympathizes with Eve. Hoping to humble Margo, Karen conspires for her to miss a performance so that Eve can perform. Eve invites the city's theater critics to attend the performance – including the acerbic Addison DeWitt. Eve's performance is a triumph. Later that night, Bill rejects Eve's attempts to seduce him. Instead, Addison takes an interest in Eve. He interviews her for a column, harshly criticizing Margo for resisting younger talent.
Margo and Bill announce their engagement at dinner with Lloyd and Karen. Eve summons Karen to the ladies' room and, after first appearing regretful, delivers an ultimatum: Karen must recommend her to Lloyd to play Cora, the lead role in Lloyd's new play. Otherwise, she will reveal Karen's part in Margo's missed performance. When Karen returns to the table, Margo surprisingly announces that she does not wish to play Cora, saying she is too old for the role.
Eve is cast as Cora. Just before the new play's premiere in New Haven, Eve reveals her next plan to Addison: to marry Lloyd, who she claims loves her, so that he can write plays for her to star in. Angered with Eve's audacity, Addison says he knows her backstory is entirely lies; her real name is Gertrude Slescynski, she was never married, and she was paid to leave town over an affair with her married boss. He also says Lloyd would never leave Karen for Eve. Addison then blackmails Eve, saying she now "belongs" to him.
Months later, Eve is a Broadway star headed for Hollywood. While accepting an award at a banquet, she thanks Margo, Bill, Lloyd, and Karen as all four coldly stare back. Eve skips the after-party and returns home where she encounters Phoebe, a teenage fan who slipped into her apartment and fell asleep. Phoebe professes her adoration and tries ingratiating herself with Eve, then begins packing her trunk. Eve invites her to stay over rather than take the long subway ride back to Brooklyn. While Eve is resting, Addison brings Eve's award to the door and is greeted by Phoebe, who admits she chose her own name. Addison realizes Phoebe will do to Eve what Eve did to Margo. When she is alone, Phoebe puts on Eve's elegant cloak and poses in front of a floor-length mirror, holding the award and bowing.
