New York City Police Department Detective Mark McPherson is investigating the murder of a young, beautiful, highly successful advertising executive, Laura Hunt, killed by a shotgun blast to the face just inside the doorway of her apartment. He first interviews charismatic newspaper columnist Waldo Lydecker, an imperious, effete older man who relates how he met Laura and became her mentor. She had become his platonic friend and steady companion, and, wishing more, he used his considerable fame, influence, and connections to advance her career.
McPherson also questions Laura's parasitic playboy fiancé, Shelby Carpenter, a "kept man" tethered to her wealthy socialite aunt, Ann Treadwell. Treadwell tolerates Shelby's infatuation with Hunt, apparently out of her practical acceptance of Carpenter's need for the affection of a woman closer to his age. Bessie Clary, Laura's loyal and highly distraught housekeeper, is also questioned by McPherson. Through the testimony of Laura's friends and reading her letters and diary, McPherson becomes obsessed with her – so much so that Lydecker finally accuses him of falling in love with the dead woman. He also learns that Lydecker was jealous of Laura's suitors, using his newspaper column and influence to keep them at bay.
One night, the detective falls asleep in Laura's apartment in front of her portrait. He is awakened by a woman entering with her own key and is shocked that it is Laura. She finds a dress in her closet that belonged to one of her models, Diane Redfern. McPherson concludes that the body assumed to have been Laura was Redfern, drawn there for a liaison by Carpenter while Laura was away in the country. With Laura still alive, unmasking the killer becomes even more urgent.
At a party celebrating Laura's return, McPherson arrests Laura for the murder of Redfern. Upon questioning her, he becomes convinced that she is innocent and that she does not love Carpenter. He searches Lydecker's apartment, where he becomes suspicious of a clock identical to the one in Laura's apartment. On closer examination, he finds it has a secret compartment. McPherson returns to Laura's apartment. Lydecker is there and notices a growing bond between Laura and the detective. Lydecker insults McPherson and is sent away by Laura but pauses on the stairwell outside. McPherson examines Laura's clock and finds the shotgun that killed Diane but leaves it there. Laura is confronted with the truth that Lydecker is the murderer.
McPherson and Laura kiss, and then he locks her into her apartment, warning her to admit no one. After he leaves, Lydecker, who had slipped in by the kitchen entrance, retrieves the shotgun, and attempts to kill Laura, saying that if he cannot have her, no one can. She deflects his shot and flees just as McPherson arrives. McPherson's sergeant shoots down Lydecker. As he dies, Lydecker whispers, "Goodbye, Laura. Goodbye, my love."
