Margo Channing (Bette Davis) is one of the biggest stars on Broadway. But having just turned forty she is worried about what her advancing age will mean for her career. After a performance of Margo’s latest play, Aged in Wood, Margo’s close friend Karen Richards (Celeste Holm), wife of the play’s author Lloyd Richards (Hugh Marlowe), brings in a besotted fan, Eve Harrington (Anne Baxter), to meet Margo. Eve tells the group gathered in Margo’s dressing room—Karen, Lloyd, Margo’s boyfriend Bill Sampson (Gary Merrill), a director who is eight years her junior, and Margo’s maid Birdie (Thelma Ritter)—that she followed Margo’s last theatrical tour to New York 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. Moved, Margo quickly befriends Eve, takes her into her home, and hires her as her assistant, leaving Birdie, who instinctively dislikes Eve, feeling put out.
Eve quickly manipulates her way into Margo’s life, acting as her secretary and adoring fan. She seems to anticipate Margo’s every need, including placing a long-distance phone call to Bill when Margo forgets his birthday. Margo becomes increasingly distrustful and bitter towards her, particularly after she catches Eve taking a bow to an empty theatre while pretending to wear Margo’s costume for Aged in Wood. Margo asks her producer, Max Fabian, to hire Eve at his office, but instead Eve manages to become Margo’s understudy without Margo’s knowledge. As Margo’s irritation grows, Karen feels sorry for Eve. In hopes of humbling Margo, Karen arranges for her to miss a performance of Aged in Wood, so Eve will have to give the performance in her place. Eve invites the city’s theatre critics, including the acerbic Addison DeWitt (George Sanders), to attend that evening’s performance, which is a triumph for her. After that evening’s performance, Eve tries to seduce Bill, but he rejects her. Instead, Addison takes her under his wing and interviews her for a column that criticizes Margo for not making way for new talents like Eve. Margo and Karen are furious.
That evening, Margo and Bill announce their engagement at dinner with the Richardses in the Cub Room of the Stork Club. Eve, who was dining at a nearby table with DeWitt, calls Karen into the ladies’ room and, after first appearing regretful, tells her to either ask Lloyd to give her the part of Cora—the lead in Lloyd’s next play, Footsteps on the Ceiling—or she will reveal Karen’s role in Margo’s missed performance. Before Karen can talk with Lloyd, Margo announces to everyone’s surprise that she does not wish to play Cora and would prefer to continue in Aged in Wood.
Eve is cast as Cora. Just before the out-of-town premiere of Footsteps on the Ceiling at the Shubert in New Haven, Eve presents Addison with her next plan: to marry Lloyd, who, she claims, has come to her professing his love and his eagerness to leave his wife for her. Now, Eve exults, Lloyd will write brilliant plays showcasing her. Angered that Eve believes she can manipulate him as easily as she does everyone else, Addison reveals he knows that her back story is all lies. Her real name is Gertrude Slescynski, she was never married, and she had been paid to leave town over an affair with her boss. Addison blackmails Eve, informing her that she will not be marrying Lloyd or anyone else; in exchange for Addison’s silence, she now ‘belongs’ to him.
Several months later, Eve is a shining Broadway star headed for Hollywood. At an awards banquet, she thanks Margo, Bill, Lloyd, and Karen with characteristic effusion, while all four stare back at her coldly. Eve skips a party in her honor, and returns home alone, where she encounters Phoebe (Barbara Bates)—a high-school-aged fan—who has slipped into her apartment and fallen asleep. The young girl professes her adoration and begins at once to insinuate herself into Eve’s life, offering to pack Eve’s trunk for Hollywood. While Eve rests in the other room, Phoebe dons the elegant robe that Eve wore to the banquet and poses in front of a multi-paned mirror, holding the award as if it were a crown.