San Francisco. Police detective John Ferguson—whom everybody calls Scottie—is actively engaged in tracking a suspect across the rooftops of buildings. At one point Scottie nearly falls to his death, precariously hanging on for dear life when a fellow cop tries to help him up. That cop does wind up falling to his death and Scottie is so traumatized by the series of events that he must retire prematurely, forced to use a cane when he walks, committed to a back brace and overcome with vertigo whenever the height gets the least bit too much.
Retirement for Scottie Ferguson means palling around with his friend—and former girlfriend—Midge. Boredom is on the verge of becoming excruciating when he hears from an old chum he went to college with: Gavin Elster. Elster tells him a story about his wife Madeine who seems to be suffering from psychological problems herself. According to Elster, his wife is prone to disappearing for days at a time as well as suddenly going into states comparable to trance. Madeleine, it seems, has come to believe that she is being possessed by the ghost of Carlotta Valdes, her own great-grandmother, no less!
Scottie remains dubious even as he agrees to help Elser by trailing his wife. As he does so, he is eventually forced to admit that her behavior is eccentric to say the least. The stunningly beautiful blonde will lapse into a near-catatonic state of staring off into space and the just as suddenly disappear just as her husband described.
What Elster did not describe was a propensity for trying to commit suicide, but that appears to be exactly what the mysteriously alluring woman does while Scottie is furtively watching one day. While looking out over San Francisco Bay, Madeleine suddenly jumps into the water. Scottie rushes in to save her, but she later claims to have absolutely no memory of doing so.
Scottie has already by then become obsessed with Elster’s wife, it not necessary in love. He is also troubled by her strange behavior and wants to help her figure out what is really at the bottom of it. Midge assists in locating a historian who fills them in on the tragic life of Carlotta who eventually committed suicide after being seduced and jilted. Madeleine grows closer to Scottie who is growing increasingly obsessed with his inability to relieve her torment. This may be due to Madeleine’s continued insistence that Carlotta is possessing her. This belief culminates in Madeleine inexorably being drawn to the high bell tower overlooking Mission San Juan Bautista. Fearing she will again try to commit suicide, Scottie chases after her, but the height proves too much and he becomes utterly paralyzed by the onset of his vertigo. He can only watch in horrified disbelief as he becomes responsible for a second person falling to their death.
His recovery from this second tragedy is slowed by a paralysis of spirit diagnosed as acute melancholia that stems directly from a guilty conscience. Midge comes to the hospital but is unable to reach into his darkness to make contact.
Upon his release from the hospital, Scottie begins imagining that he sees Madeleine everywhere that the two of visited together. In his seeming madness, eventually every beautiful blonde carries the potential of being Madeleine. One day, the potential becomes too great to bear and too complete to be mere coincidence. The only problem is that this girl who could almost be Madeleine’s identical twin is a brunette.
When he follows her home one day, he discovers her name is Judy Barton. At least, that’s what she says, but he refuses to believe her, convinced she must somehow be Madeleine. He invites her to dinner and she reluctantly says yes. As Scottie exits, a voiceover offers explanation for what is happening in the form of a letter Judy writes to Scottie.
She was hired by Elster to play the part of his wife as part of an intricate plot to kill the real Madeleine and then cover it up. The story of Carlotta’s ghost was just another level of the fictional cover story. By the time the body that Scottie witnessed falling off the bell tower was thrown off, Madeleine was already dead. The scream from above came from Judy. Making the entire situation almost unbearable is the fact that as Scottie was pursuing Madeleine, Judy was falling for Scottie.
Tearing the letter into pieces, Judy decides to attempt to get Scottie to fall in love with her as herself. They begin seeing each other, but her plans simply isn’t working. With each passing day, Scottie goes to greater lengths to transform the brunette Judy into the blonde Madeleine he fell in love with, still believing she is the woman he witnessed falling to her death. Judy’s dark hair is dyed to the perfectly match tone of Madeleine’s blonde mane. He buys her the same outfits worn by Madeleine. The obsession has now turned a corner and is taking them both down a very dark road of fetishistic fixation. Judy becomes increasingly uncomfortable, but continues down that road alongside Scottie.
Then she makes a fatal error in judgment. She puts on a necklace which Scottie recognizes as one that had been passed down to Madeleine from Carlotta. Finally, Scottie breaks out of his own trancelike mania to turn Judy into a carbon copy of a dead woman and realizes that he is at the center of a con. Confusion infused with anger and bewilderment leads Scottie to drag Judy to the same mission where Madeleine died. They make their way up the steps of the bell tower and there forces a full admission from Judy.
Judy tells Scottie that Elster purposely sought him out for the plot to take advantage of his fear of paralyzing vertigo. Elster knew there was no possibility of Scottie getting to the top of the bell tower where the murder would take place. Heartbroken at what part she’s played in destroying him, she begs Scottie to love her…Judy. And to keep her safe. This time they both reach the top of the bell tower, but a nun who has been alarmed by the commotion begins to investigate the tower. Gripped by her own overwhelming guilt and the sense of confusion brought on by Scottie’s obsession with her transformation, all the pent-up fear that Judy has been holding in overwhelms her at the sound of the nun’s approach and she jumps over the ledge to her death down below.
On the verge of madness at the third such death he has not only witnessed, but shares responsibility for, Scottie can do nothing but stand there paralyzed, looking down at Judy’s lifeless body.