The film begins with the sound of a typewriter - and the words of The Fan, Douglas Breen, dictating along as he types out a letter to Sally Ross, an ageing film star. He tells her that he is he greatest fan and asks her to write back to him enclosing a recent photo.
We see from an envelope on his desk that this is New York City:
The story cuts to the Morosco Theater were Sally Ross is surrounded by fans who have come to see her following her latest show called It's Called Tomorrow. A female fan rips the pen from Ross's fingers and runs across the busy street, almost getting hit by the cars. On the other side of the street she is tripped up. It is Breen. He reaches down and takes the pen, giving the girl a nasty glare. The girl gets up and runs away.
The next scene flits back and forth between these two lonely people. Douglas Breen is moving around in his apartment and Sally Ross is getting undressed in her apartment at the Prasada Hotel.
Some time later, Breen receives a response to his fan letter but it is a stock response from Ross's secretary, Belle Goldman. He writes back instantly, upset that Ross has not responded personally and finishes with. PS. that he already has that particular photo.
The scene cuts to Belle Goldman who is working in a small office area within Ross's apartment. She is opening all the post, dealing with all the fan letters and other business. She reads Breen's latest letter and remarks on it, but Ross is not interested. Belle sends back a kind response and encloses a newer photo.
Despite her kind words, Breen is upset when he receives yet another letter from the secretary rather than from the star herself. He writes back with a veiled threat, saying that her rudeness would not go unpunished.
Sally Ross is lonely; she is approaching 50 rapidly and is starting to feel her age. She meets her ex-husband, Jake Berman, for lunch, a man she still loves, only to hear from him that he is intending to marry the new woman in his life - Heidi.
More of Breen's character is revealed in the next scene where we see him arriving at his job at Record City. The manager orders him to get to work, but Breen notices that his female colleague is late. When he is informed that she has a good reason for her absence he gets angry especially as she is very smug about being given what seems to be preferential treatment. Breen, deliberately, drops a pile of records in her path.
That night there is a movie on television starring Sally Ross and Breen has set up a candle-lit dinner in some fantasy of sharing dinner with her. It is interrupted by a knock on the door. Breen is angry. He berates his sister who has come to see him as she and their parents are worried about his lack of contact with them. When he says he is having dinner with a great actress, she tells him he is delusional and he slams the door in her face.
Sally Ross has accepted the lead role in a Broadway musical called 'Never Say Never'. She is worried when she attends the rehearsals, convinced that she might be too old for the role. She soon gets into the swing of it and starts to enjoy herself.
Meantime, Breen is escalating. He writes to Ross, saying that he knows the secretary is deliberately keeping them apart, calls Belle a lesbian in his letter. He has delusions, believes that they were created for one another and goes into great detail on the erotic fantasy concerning him and Sally Ross. He takes the letter, personally, to Jo Jo's Factory - where the rehearsals for the musical are being held - and hands it to one of the male dancers. However, the man hands it straight to Belle. In a fit of anger he stalks out and heads across the street to a store that is selling knives, selecting a cutthroat razor.
The next scene shows Sally Ross getting ready for a party. She has an angry scene with Belle after hearing about the latest letter and the accusation inside that Belle is getting between Ross and her ardent fans. She accuses Belle of not doing her job properly. They make up - and then Ross's escort for the party arrives. It is David Barnum, a young male dancer from the musical - dressed in a tuxedo. Ross's ex-husband, a movie star in his own right, is at the party with his soon-to-be fiancée and there is an awkward moment.
In the meantime, Belle has gone shopping before heading home. Breen is sitting in one of the chairs reserved for the shoeshine, waiting. He follows her along the deserted corridors of the Metro then grabs her. He slashes her face, twice, with the razor.
Jake Berman goes to the hospital to give Sally some support as she waits to hear news on Belle. Afterwards, she goes back to her hotel apartment. She opens a letter, from Breen, which says that he was the one who slashed Belle. The letter is full of sexual comments and a demand that she writes back immediately. Unfortunately, there is no return address and he has signed with only his first name. The letter is handed to Police Inspector Raphael Andrews.
Andrews is convinced that this fan is psychotic, and that he is a potential killer.
Meanwhile, Breen is seated in the café opposite. He sees Sally Ross leaving and watches as David Barnum approaches her, gives her a hug and a kiss on the cheek. Breen is angry. He follows Barnum to the West Side YMCA, strips off and slides into the swimming pool wearing only his boxers. Barnum is swimming lengths of the pool. Breen swims underwater, beneath Barnum and slashes at him from sternum to groin.
We learn that David is alive but in a stable condition when Ross has dinner with Inspector Andrews.
Later, Breen has managed to get into Ross's apartment, into her bedroom. Ross's housemaid, Elsa, who has been taking the phone calls since Belle was hospitalised, goes into the bedroom to hang up some clothes. Breen attacks her and he slices her across the chest with the razor, leaving her lying in a pool of blood. In anger, he starts to slice and destroy Ross's apartment then writes her a letter.
Breen: Dear Bitch. See how accessible you are? How would you like to be fucked with a meat cleaver?
Later he contacts her from a call box... Sally, I want to touch you, want to hug you, want to make love-- She slams down the phone, and Breen spits at the receiver.
Ross has been given a female officer for personal protection but she slips something into the woman's bedtime drink and sneaks away. Ross goes to the old hideaway that she and Jake used. She has a moment of fear when someone drives up and starts banging on the door, but it is just a neighbour.
The scene changes. We find Breen sitting by himself in a bar with a drink in front of him. He is catching the eye of another young man, giving him coy looks. The guy takes the bait, comes over and places his beer on the counter beside Breen. Breen gets up and follows the man from the bar. They walk along the street together, enter a building and climb to the roof. Breen leans back against a walled ventilation stack. The man leans in and kisses him before sinking slowly to his knees in front of Breen. We get the impression that he is undoing Breen's pants, touching him. Meanwhile, Breen is reaching up to where he had left the razor hidden. He kills the man and then picks up a can of petrol and pours it over the body. He sets fire to the body and pins up a letter close by - a suicide note from Douglas Breen.
Inspector Andrews and other Police officers are going through Breen's apartment. He matches up the letters with the typewriter, and looks at the photos of Sally Ross covering the wall. Jake Berman goes to the 'hideaway' to tell Ross that it is over, that Douglas Breen is dead.
Everything seems to settle back down. Belle returns and starts making arrangements for house seats for Inspector Andrews and Officer Emily Stolz. Jake Berman tells Sally that Heidi has left for good, that nearly losing Sally reminded him of how good they were together. He wants to try again.
Opening night for the musical. We watch Sally Ross in snippets of her musical numbers, interspersed with Breen getting dressed up in a tuxedo. He places a red carnation in his lapel, and tidies his hair. He arrives during the performance, while she is singing the 'Hearts Not Diamonds' number, taking a seat two rows behind Jake Berman, Belle and the Inspector. He walks out after her number.
The show seems to be a success and there is an opening night party. Berman and the others go on ahead, leaving Sally Ross behind to tidy herself up. Her wardrobe mistress, Hilda, is moving around, replacing costumes.
Breen attacks Hilda, killing her. He then goes to the stage door, says Goodnight, Pop to the older man seated there, pretending to be familiar with him, then he stabs Pop. Breen locks the stage door, leaving Pop in a pool of blood. Breen confronts Ross in her dressing room. She runs away to the stage door - but it is locked. Breen taunts her with the keys. He chases her into the auditorium but she has grabbed one of the stage props, a horsewhip, and starts to beat him. He grabs the whip away and hits her with it.
'I never wanted to hurt you, Sally. I lived my whole life for you and you never answered me once. Not once. Not one letter.' Ross has fallen down; he pulls her up by the hair. 'I expected so much more from a star like you, Miss Ross.'
Breen turns gentle on her, pushes the hair from her eyes and goes to kiss him.
Ross: You're pathetic
He is angry again, he takes out the knife but she confronts him, verbally, and he hugs her.
Breen: I love you. Please love me.
She pretends to hug him back but it is a movement to conceal the fact that she is taking the razor from his fingers. She shoves the point into the side of his throat.
Last scene: Ross is standing, hunched over in shock. Near by, Breen is seated on one of the audience seats. There is a trickle of blood down his throat. His eyes are wide open, hair mussed, staring into nothing as if dead, or in a complete state of shock.