The police investigate the murder of a movie writer at the home of an old-time movie star. Six months ago, the victim, Joe Gillis, was an unemployed writer whose latest script had been rejected by Paramount, based on script reader Betty Schaefer's poor evaluation of it. Fleeing car repossessors, Joe hid his car in the garage of a run-down mansion on Sunset Boulevard. The mansion turned out to be inhabited by vain, reclusive, silent movie star Norma Desmond and her stern yet devoted butler Max. When Joe told Norma he was a screenwriter, she had him read her script for Salome, which she planned to use for her return to movies. Joe privately thought the script was awful but told Norma all her "fantastic" script needed was some editing for an expensive fee. She hired him as long as he agreed to move from his apartment to her guest house, which he did.
Joe continued to give in to Norma. His efforts to edit Norma's script were stymied by her indignation at cutting out parts featuring her. His car was located and towed away by the car repossessors. Despite his weak protests, she bought him expensive new clothes. Max moved Joe's things to the husband's bedroom in the main house, next to Norma's, ensuring a loss of privacy for Joe. Max told Joe that Norma had experienced depression and tried to commit suicide in the past. Max also implied that he wrote the fan letters Norma received everyday.
When Norma tried to get closer to Joe, they argued. Joe went to his friend Artie's party and met Artie's girlfriend Betty, the script reader at Paramount. Joe was attracted to Betty but left Artie's party in a rush when he phoned Max and found out that Norma had attempted suicide. Joe reconciled with Norma and they kissed.
Norma sent her script to director Cecil B. DeMille at Paramount, but someone other than DeMille from Paramount called her. Norma went to visit DeMille at the studio and he tried to let her down gently. DeMille's refusal didn't register with Norma, who thought he would make his next movie with her. Joe visited Betty, who proposed writing a script with him. Max learned that Paramount had called Norma because they were interested in renting her antique car, not in her, and told Joe.
A deluded Norma prepared herself for her return to movies, while Joe secretly wrote a script with Betty and grew more attracted to her. Max was concerned that Norma might discover the truth about Joe. Max told Joe he protected Norma from the truth ever since he, an ex-film director, had made her into a star. He was her first husband. He'd given up directing to work as her servant because he still loved her.
Betty told Joe she'd fallen in love with him. Norma found Joe's and Betty's script and phoned Betty to tell Betty about the truth about Joe. Joe invited Betty to the house and told her he was Norma's kept man. Betty asked him to leave with her, but he pretended he was interested in his lavish lifestyle than her. After Betty left, Joe prepared to leave Norma, armed only with his original belongings. Joe told Norma the truth: that there would be no movie with DeMille, that the studio had only been interested in her car and that Max wrote her fan mail. Norma went insane and shot Joe dead.
The police try to interrogate Norma but she's completely deluded. She thinks that the news cameras are movie cameras and that she's acting a scene. Max pretends to direct her in front of the news cameras to get her to go downstairs and into a police car. She walks downstairs and announces how happy she is to be making movies again.