A montage illustrates Norma Desmond's slavish regimen of beauty treatments under blindingly bright lights:

After that, an army of beauty experts invaded her house on Sunset Boulevard. She went through a merciless series of treatments. Like an athlete training for the Olympic games, she counted every calorie, went to bed every night at nine. She was absolutely determined to be ready, ready for those cameras that would never turn.

In the downstairs living room, as Joe reads a book titled The Young Lions, Norma suspects that he sneaks out late at night in the Desmond car. When confronted, he lies and tells her that he drove to the beach: "You don't want me to feel that I'm locked up in this house?" She begs for his understanding during a stressful period: "All I ask is for you to be a little patient and a little kind." Joe is sneaking out at night to go to Betty's office at Paramount to work on their script. [The film mirrors the script work that Joe does for Norma with the conventional scriptwriting he completes with Betty.]

Yes, I was playing hooky every evening along in there. It made me think of when I was twelve and used to sneak out on the folks to see a gangster picture. This time, it wasn't to see a picture, it was to try and write one. That story of mine Betty Schaefer had dug up kept going through my head like a dozen locomotives, so we started working on it, the two of us, nights when the studio was deserted, up in her little cubby-hole of an office.

They are framed in two separate windows: on the left, Joe paces with his coat off and shirt-sleeves rolled up; on the right, Betty sits at her desk, typing the script. In one of their clandestine meetings, he tells her how he truly enjoys their relationship: "It's fun writing with you." His opened, pure gold cigarette case with the inscription: "Mad about the Boy - Norma" catches her eye. She asks curiously: "Who's Norma?" Joe describes the woman of his anomalous relationship, with whom he plays the role of a 'kept' man because times are hard: "...a friend of mine, a middle-aged lady, very foolish and very generous."

As they work more hours together, they are joined together in the frame as they walk on the studio lot late one evening - "a little tour of the drowsing lot, not talking much, just wandering down alleys between the sound stages, or through the sets they were getting ready for the next day's shooting." On the fake set of a New York Street ("all cardboard, all hollow, all phony, all done with mirrors"), they discuss her third-generation Hollywood film roots and the $300 plastic surgery she had done on her slanted nose. She thought she might be a movie star in films, so she had a nose job, but then decided it would be okay to forego fame as an actress:

Betty: ...I used to play here when I was a kid.
Joe: What were you - a child actress?
Betty: No, I was born just two blocks from the studio, right on Lemon Grove Avenue. My father was an electrician here until he died. Mother still works in Wardrobe.
Joe: Second generation, huh?
Betty: Third. Grandma did stunt work for Pearl White. I come from a picture family. Naturally, they expected me to become a great star, so I had ten years of dramatic lessons, diction, dancing. Then the studio made a test. Well, they didn't like my nose - it slanted this way a little, so I went to a doctor and had it fixed. They made more tests and they were crazy about my nose - only they didn't like my acting...It taught me a little sense. I got a job in the mail room, worked up to the stenographic, now I'm a reader!
Joe: Come clean, Betty. At night, you weep for those lost close-ups, those gala openings!
Betty: Not once. What's wrong with being on the other side of the cameras? It's really more fun.
Joe: Three cheers for Betty Schaefer.

[Betty is realistic about the importance of illusion in the Hollywood system, in contrast to Joe's problems with reality in the industry.] She is level-headed and sensible, and claims that it's really "more fun" to be on the other side of the cameras than being in the spotlight. Joe affectionately kisses her nose. He compares her to a youthful, non-neurotic or tarnished "smart girl" - a "brand new automobile" - something that he is literally lacking and needs to acquire as a symbol of success:

Joe: May I say that you smell really special?
Betty: It must be my new shampoo.
Joe: That's no shampoo. It's more like freshly-laundered linen handkerchiefs, like a brand new automobile. How old are you anyway?
Betty: Twenty-two.
Joe: Smart girl. Nothing like being twenty-two. And may I suggest that if we're ever to finish this story, you stay at least two feet away from me. Now the first time you see me coming any closer, I want you to take off a shoe and clunk me on the head with it.

He asks her help in restraining his impulses, and then directs them to go "back to the typewriters, by way of Washington Square." [Joe is referring to walking through the sets used for director William Wyler's The Heiress (1949), based on Henry James' novel Washington Square.]

On his return to the mansion, Max meets him in the shadows of the garage. Protectively, he tells Joe to be careful so that Norma will not see him coming home so late. Max is concerned that Norma has become pathologically suspicious - she begun to notice how often Joe is absent. Joe asks Max how long he is going to shield Norma from the truth that he is writing a script with Betty. Joe is not interested in hiding the truth from Norma any more, although Max explains why he has dutifully accepted his subservient role:

Joe: And we're not helping her any, feeding her lies and more lies. Getting herself ready for a picture. What happens when she finds out?
Max: She never will. That is my job and it has been for a long time. You must understand, I discovered her when she was sixteen. I made her a star and I cannot let her be destroyed...I directed all her early films. There were three young directors who showed promise in those days: D. W. Griffith, Cecil B. De Mille, and Max von Mayerling. [Max von Mayerling is actually Erich von Stroheim.]
Joe (cynically): And she's turned you into a servant.
Max (revealing why he came back): It was I who asked to come back, humiliating as it may seem. I could have continued my career, only I found everything unendurable after she had left me. You see, I was her first husband.

More tormented than ever, Norma suspects Joe's dalliance with a woman. She becomes aware of Joe's relationship with Betty when she sees their names as co-writers on the title page of their script. In the light, the typewritten script credits are revealed, in a zooming close-up:

UNTITLED LOVE STORY by JOSEPH C. GILLIS and BETTY SCHAEFER

The title, a symbolic representation of the couple's budding relationship, is threatening to Norma.

At the same time during one of their writing sessions - to make matters worse - Betty betrays her real feelings. She tearfully tells Joe that she is no longer in love with her fiancee, Artie Green, even though he telegrammed her to come to Arizona to get married.

Joe: You're getting married. That's what you wanted.
Betty: I don't want it now.
Joe: Why not? Don't you love Artie?
Betty: Of course I love him. I always will. I'm not in love with him any more, that's all.
Joe: What happened?
Betty: You did. (He takes her in his arms and they kiss, obviously in love.)

Feeling like a "heel" for breaking up his best friend's relationship and for being dishonest, Joe nonetheless is "crazy about" Betty and ponders how he can "get away with it."

There it was - Betty Schaefer's future right in the palm of my hand. Betty Schaefer engaged to Artie Green, as nice a guy as ever lived. And she was in love with me. Me! She was a fool not to sense that there was something phony in my set-up. And I was a heel not to have told her. But you just can't say those things to somebody you're crazy about. Maybe I'd never have to. Maybe I could get away with it, get away from Norma. Maybe I could wipe the whole nasty mess right out of my life.

Jealous of the couple, a half-mad Norma tries to break up their relationship. She phones Betty and asks her in a concealed voice about the reality of Joe's setup - as a gigolo. She explains that she's doing the young girl a favor by sparing her "a great deal of misery":

Exactly how much do you know about him? Do you know where he lives? Do you know how he lives? Do you know what he lives on?

She hints that Joe is manipulatively two-timing her, just as Joe walks in from the door behind her and discovers what Norma is doing. After he angrily grabs the phone away, he invites Betty to come out and see for herself at his address: 10086 Sunset Boulevard. In a stand-off while waiting for Betty to arrive, the pitiable silent screen actress tells Joe that she has a revolver but didn't have the courage to kill herself. She begs him as she squirms on her bed: "Say you don't hate me, Joe." When the doorbell rings signaling Betty's arrival, Norma rises up, moving the pillow behind her to reveal the revolver: "What are you going to do, Joe? What are you going to do?"

Joe sarcastically explains to Betty how he has been a 'kept man' as he shows her around the extravagant mansion (one of the "old Hollywood palazzos"). He describes his living arrangement with Norma - a relationship in which he sold his self-respect for wealth and its accouterments:

This is an enormous place...It's lonely here, so she got herself a companion. A very simple set-up. An older woman who is well-to-do and a younger man who is not doing too well. Can you figure it out yourself?

Betty realizes his predicament - she tells him that if he loves her, he should get his things together and get out of there with her. She doesn't want to hear any more. He refuses to join her and explains that he must stay with Norma - with his "eighteen suits, and all my custom-made shoes and the six dozen shirts, and the cuff-links and the platinum keychains, and the cigarette cases," rather than return to a "one-room apartment that I can't pay for":

Look sweetie, be practical. I've got a good deal here. A long-term contract with no options. I like it that way. Maybe it's not very admirable. Well, you and Artie can be admirable.

With her eyes filled with tears, Betty leaves by herself in the middle of the night after he has wished her good luck at the front door: "You can finish that script on the way to Arizona."

When Joe turns back toward the house, Norma is perched behind the balustrade above him, peering down. Joe goes upstairs to his bedroom to pack his suitcase, silently passing by Norma and preparing to leave Hollywood altogether for his old newspaper copy desk position in Ohio. He rejects further romance with both Betty and Norma - two women who represent different facets of the falsifying Hollywood experience.

In front of a mirror next to his bedroom door, Norma peels off patches from her face and readjusts her hair and appearance, assuring him: "I've stopped crying. I'm all right again. Joe, tell me you're not cross. Tell me everything is just as it was, Joe." He returns all the gifts of clothing and other "trinkets" and throws some of the jewelry items on the bed. and then summarizes his feelings: "I don't qualify for the job, not any more." Norma calls frantically for Max and threatens to shoot herself with a gun when she realizes he is leaving her - she lapses into madness and cries:

I can't face life without you, and you know I'm not afraid to die.

Joe tries to convince her that her fans have left her and to face up to reality that hundreds of thousands of people won't care if she kills herself:

Oh, wake up, Norma. You'd be killing yourself to an empty house. The audience left twenty years ago. Now face it.

He claims that her dependence on him as the "Salome" scriptwriter and on director De Mille to further her creative career were both foolhardy. He confesses that Paramount Studios and director De Mille didn't really want her to star in a production; her script was unfilmable; and the studio was only interested in her vintage car for rental. And when Max enters, he reveals that her fan mail was not real: "Tell her there isn't going to be any picture, there aren't any fan letters except the ones you write." Max steadfastly defends Norma: "Madame is the greatest star of them all." Joe tries to get the aging and suicidal Norma to act her age: "Norma, you're a woman of 50. Now, grow up! There's nothing tragic about being 50 - not unless you try to be 25." Norma repeatedly deludes herself: "I'm a star...I'm the greatest star of them all." As he heads for the door, she cannot bear to be deserted and have him leave her presence, hissing: "No one ever leaves a star. That's what makes one a star."

As he walks away from her down the staircase and toward the outdoor pool, she shoots him once - and then two more times with the gun that she had obtained for her own suicide. No longer a victim of her own self-hatred, she outwardly vents her fury on him for abandoning her - like so many others in the audiences or studios of the talkies. [Symbolically, she also liberates herself by taking revenge on those who exploited and 'robbed' her during (and after) her Hollywood acting career.] He staggers, turns and falls into the luminous pool, mortally wounded. When Max reaches Norma's side, she exultantly says to him, looking up at the stars in the night sky: "Stars are ageless, aren't they?"

The scene dissolves into an underwater shot looking upward at Joe's spread-eagled body floating in the filled swimming pool - his watery grave into the next morning. [In the film's opening, Joe's posthumous voice was that of the narrator - he is the luckless dead victim floating in the swimming pool.]

The unlucky screenwriter 'ghostwrites' his narrated story up to the present time:

Well, this is where you came in. Back at that pool again, the one I always wanted. It's dawn now, and they must have photographed me a thousand times. Then they got a couple of pruning hooks from the garden and fished me out, ever so gently. Funny how gentle people get with you once you're dead. They beached me like a harpooned baby whale and started to check the damage, just for the record.

As his body is fished out of the pool, placed on a stretcher, and carried toward the coroner's vehicle, and the Desmond house is filled with police officers, newspaper and tabloid reporters, photographers and other journalists, Joe anticipates how the headlines would devastate Norma following his death:

...the whole joint was jumping - cops, reporters, neighbors, passersby, as much hoopdedoo as we get in Los Angeles when they open a Super Market. Even the newsreel guys came roaring in [ironically, from Paramount Pictures News Division]. Here was an item everybody could have some fun with. The heartless so-and-sos! What would they do to Norma? Even if she got away with it in court - crime of passion, temporary insanity - those headlines would kill her: 'Forgotten Star a Slayer,' 'Aging Actress,' 'Yesterday's Glamour Queen'...

The Lieutenant from the Homicide Bureau must wait to use the single phone line in the house. Hollywood gossip columnist Hedda Hopper (playing herself) insists that her story is "more important" and monopolizes the white telephone in Norma's upstairs bedroom to call in her story about Norma's madness:

Times City Desk? Hedda Hopper speaking. I'm talking from the bedroom of Norma Desmond. Don't bother with a rewrite man. Take it direct. Ready? 'As day breaks over the murder house, Norma Desmond, famous star of yesteryear, is in a state of complete mental shock. A curtain of silence seems to have fallen around her. She sits in the silken boudoir of her house...'

While being interrogated by local police and the LA Homicide Squad, a dazed Norma ignores them and stares blankly at her image in a mirror. She responds only after learning that newsmen with cameras have arrived:

Norma: Cameras? What is it, Max?
Max: The cameras have arrived.
Norma: They have? Tell Mr. De Mille I'll be on the set at once.

The crazed and deluded woman is persuaded and coaxed to go quietly downstairs to a waiting car through a group of assembled reporters and cameramen - to surrender, only by being made to think that she is experiencing her longed-for return and shooting a film scene for famous movie director Cecil B. De Mille. Norma runs her fingers through her hair and applies makeup powder to get ready for her key scene. Max prepares the newsreel cameramen at the foot of the staircase. Norma emerges from her bedroom, draped in a sparkling robe with golden spangles in her hair. Then, looking up toward the second floor balcony where Norma will enter the scene, Max shouts: "Quiet everybody!...Lights!" But first, he must encouragingly explain the scene to a confused, disoriented Norma and direct the news cameras to roll:

Max: Are you ready, Norma?
Norma: What is the scene? Where am I?
Max: This is the staircase of the palace.
Norma: (readying herself) Oh yes, yes. Down below, they're waiting for the princess. I'm ready.
Max: (shouting) All right. Cameras! Action!

Sweeping her gown around with one hand, she begins to descend the mansion's staircase for her dramatic entrance and final close-up as the main focus of the big scene, while everyone is awestruck by her sinuous performance. Joe narrates, introducing her ironic, victorious comeback down the marble staircase in her decaying Hollywood mansion to the whir of cameras and the radiant lights of the popping flashbulbs:

So they were turning, after all - those cameras. Life, which can be strangely merciful, had taken pity on Norma Desmond. The dream she had clung to so desperately had enfolded her.

In the memorable conclusion of her grand entrance, the delusional Norma Desmond descends the marble staircase believing she is playing Salome in the most important scene of her career. At the bottom of the stairs in a closeup (exhibiting her arched eyebrows, glaring eyes, and oddly-twisted, insane-looking facial expression), she has become so overjoyed and moved that she has to pause and have a word for the crew - and for her audience (both around her and the "people out there in the dark" - the moviegoers who had abandoned her and the silent-era performers, or any performers that had outlived their fame):

I can't go on with the scene. I'm too happy! Mr. De Mille, do you mind if I say a few words? Thank you. I just want to tell you all how happy I am to be back in the studio making a picture again! You don't know how much I've missed all of you. And I promise you, I'll never desert you again because after Salome we'll make another picture, and another picture! You see, this is my life. It always will be! (In a whisper) There's nothing else - just us - and the cameras - and those wonderful people out there in the dark. All right, Mr. De Mille, I'm ready for my close-up.

Then the boundaries between reality and dreams shift as Norma walks directly toward and past the offscreen newsreel cameras filming the scene. She fulfills her perverse, illusory dream to be a star. As one camera closes in on her face, her image goes into a blurry soft-focus, as Norma slips transcendently backward in time to her glory days - a time of illusion that has passed forever.