At a mansion on Sunset Boulevard, police officers and photographers discover the body of Joe Gillis floating face down in the swimming pool. In a flashback, Joe relates the events leading to his death.
Six months earlier, Joe is a down-on-his-luck screenwriter trying to interest Paramount Pictures in a story he submitted. Script reader Betty Schaefer harshly critiques it, unaware that Joe is listening. Later, while fleeing from repo men seeking his car, Joe turns into the driveway of a seemingly deserted mansion inhabited by forgotten silent film star Norma Desmond. Learning that Joe is a writer, Norma asks his opinion of a script she has written for a film about Salome. She plans to play the role herself in her return to the screen. Joe finds her script abysmal, but flatters her into hiring him as a script doctor.
Joe moves into Norma's mansion at her insistence, and sees that Norma refuses to believe that her fame has evaporated. Her butler Max secretly writes all of the fan mail that she receives in order to maintain the illusion. At her New Year's Eve party, Joe realizes that she has fallen in love with him. He tries to let her down gently, but Norma slaps him and retreats to her room, distraught. Joe visits his friend Artie Green and again meets Betty, who thinks a scene in one of Joe's scripts has potential. When he phones Max to have him pack his things, Max tells him Norma has cut her wrists with his razor. Joe then returns to Norma, and their relationship becomes intimate.
Norma has Max deliver the edited Salome script to her former director Cecil B. DeMille at Paramount. She starts getting calls from Paramount executive Gordon Cole but refuses to speak to anyone except DeMille. Eventually, she has Max drive her and Joe to Paramount in her 1929 Isotta Fraschini. DeMille welcomes her affectionately and treats her with great respect, but tactfully evades her questions about the script. Max then learns that Cole only called her because he wants to rent her Isotta Fraschini for use in a film.
Preparing for her imagined comeback, Norma undergoes rigorous beauty treatments. Joe secretly works nights in Betty's office, collaborating on an original screenplay. His moonlighting is discovered by Max, who then reveals that he was once a respected film director who discovered Norma, made her a star, and was her first husband. After she divorced him, he abandoned his career to become her servant.
Norma discovers a manuscript with Joe and Betty's names on it and phones Betty, insinuating that Joe is not the man he seems. Overhearing the call, Joe invites Betty to the mansion to see for herself. When she arrives, he pretends that he is satisfied being a gigolo so that she can be with Artie. However, after she tearfully leaves, he packs to return to his old newspaper job in Dayton, Ohio. He bluntly informs Norma that there will be no comeback, that Max writes all of her fan mail, and that she has been forgotten. He disregards Norma's threat to kill herself as she brandishes a gun; as he leaves the house, Norma shoots him three times, and he collapses into the pool.
The flashback ends and the film returns to the present day, with Desmond about to be arrested for murder. The mansion is overrun with police and reporters with newsreel cameras, which she believes are film cameras. Max pretends to "direct" her, and the police play along. As the cameras roll, Norma descends the grand staircase for what she thinks is a close-up for DeMille. She stops and makes an impromptu speech about how happy she is to be making a film again, then walks towards the camera.
