Douglas Breen, a deranged young New York City record salesman, writes a rambling letter to stage and film star Sally Ross. Sally’s assistant, Belle Goldman, has been intercepting Douglas’s numerous disturbed letters, responding herself and asking him to stop. Douglas feels ignored, and becomes determined to meet with Sally and consummate “his love” for her.
Sally has recently taken a job in a musical stage production, and has been reconnecting with her ex-husband, Jake Berman, who has arrived from California to film a movie. After Belle receives another explicit letter from Douglas, she brings it to Sally’s attention; Sally scolds her for being rude to the fan, and brushes it off, saying she has had to deal with many fans who have had extensive “fantasies” about her. Meanwhile, Douglas begins stalking Sally, sitting outside of her apartment building, and following her to her stage rehearsals. He decides to hand-deliver a letter to her while she is rehearsing for the musical, but watches the man at the studio give the letter to Belle, whom he realizes is the assistant who has been writing the nasty responses to him.
After the rehearsal ends, Douglas follows Belle into the subway, where he slashes her face open with a razor. She survives the attack, but is hospitalized. When police press her for information, Belle cannot recall the return addresses written on the obsessive letters to Sally, and Sally informs them that she does not keep the fan mail she receives. Increasingly enraged by his lack of contact with Sally, Douglas manages to break into her apartment and murders her maid, Elsa, in the bathroom. Sally returns home with a private investigator, and finds Elsa’s body in a pool of blood, and her apartment in shambles. A threatening letter addressed to Sally is left behind, reading: “Dearest bitch, See how accessible you are? How would you liked to be fucked with a meat cleaver?”
Sally, distraught, flees New York and retreats to a secluded house in the country, where she is visited by Jake. Meanwhile, at a bar, Douglas meets a man who cruises him for sex. The two go to the rooftop of Douglas’s building, where the man begins to perform oral sex on him, but Douglas stabs him to death and lights his body on fire. Douglas leaves a suicide letter with the body in an attempt to lead police to believe the body is his own, and that he took his own life.
The opening night of the musical arrives, and Sally reluctantly returns to the city to perform. Douglas sits in the audience, watching her. After the show, Sally sits in her dressing room with the costume designer, Hilda. Douglas kills both Hilda and a nightwatchman while Sally removes her makeup. He confronts Sally in her dressing room, covered in blood, and chases her through the empty theater. She strikes him in the face with a riding crop. Douglas slaps her across the face, throwing her to the floor, and beats her with the crop. As he tries to kiss her, Sally tells him he is pathetic. His rage subsides, and he embraces Sally, begging her to love him. As he holds her, she plunges his knife into his neck. He collapses, landing in one of the theatre’s chairs, the knife still in his neck. Sally leaves the theatre, and a voiceover of Douglas’s first letter to her plays. In it, he says:
Dear Miss Ross, I have finally worked up enough courage to write you. You do not know me, but who I am does not matter. If there is such a thing as a soul, which is the basis of all life…then you are my soul. And your life is my life. This is the first letter of what I hope will be an everlasting correspondence. Your greatest fan, Douglas Breen.