Lights, camera, action!
Meet Joe Gillis. He's dead. And floating in a pool.
How did this happen? Well, his disembodied voice leads us back in time and gives us the skinny.
Joe is a screenwriter, trying to make car payments and expenses. Unable to find work or get a loan from his movie business friends, he decides to quit L.A. and go back to Ohio—but two repo men chase him before he can leave. He hides his car in a driveway to a mansion, which appears abandoned. But, in fact, it belongs to Norma Desmond, a former silent era movie star who is extremely proud and narcissistic and just shy of stone-cold crazy.
Norma ends up letting him stay at her place after he offers to help edit the screenplay she's been writing. Norma's servant, Max, explains the world within this "grim sunset castle" to Joe, and turns him into a live-in guest at the mansion. Norma gives Joe gifts and gradually falls in love with him, until she throws a New Year's Eve party and openly tries to make him hers once and for all. Joe refuses and runs off to another party, but is forced to return when Norma makes a suicide attempt. He relents and sleeps with her, becoming her kept-man.
But Joe is restless to escape. While Norma labors under the delusion that the famous (in real life) director Cecil B. DeMille will be making her screenplay into a movie—even visiting DeMille at Paramount—Joe secretly starts working on a screenplay with Betty Schaeffer, a script reader and the girlfriend of one of his friends, Artie. Joe and Betty start to fall in love, but Norma discovers their screenplay and catches the jealousy bug. She phones Betty and hints at the true nature of her relationship with Joe. Joe invites Betty over and tells her the truth, asking her to leave and marry Artie and have a nice life.
Norma thinks she's won.
Not so much.
Joe plans to leave her, too, and head back to the simpler life he left in Dayton, Ohio. Before he goes, he tries to tell Norma that there's nothing wrong with getting old and losing her celebrity, and that Cecil B. DeMille isn't really making her script into a movie. When he tries to leave, Norma murders him with a revolver, leaving his body in the swimming pool where it fell. The next day, reporters and policemen arrive, and Norma flies into total madness, thinking she's acting in her movie directed by DeMille. She ends by performing a deranged looking close-up for the assembled news cameras at the crime scene...
...And we all have nightmares for many nights to come.