New York police detective Mark McPherson is assigned to the case and he begins his investigation by questioning Waldo Lydecker, a well known columnist and radio personality. Lydecker was a close friend and confidante of Laura, and tells McPherson that he 'created' her. Having encountered Laura as a struggling ad writer, he became fascinated by her and undertook to make her a success in her career and in society. He advised her on how to dress and comport herself, introduced her to all the right people, and promoted her in his column. A lot of the first part of the movie is told in flashbacks narrated by Lydecker.
Through these interviews, and Lydecker's commentary, and under the haunting eyes of Laura's portrait, McPherson slowly becomes fascinated- or more accurately, obsessed- with Laura. At one point, Lydecker snidely comments on this, accusing McPherson of falling in love with a corpse. This idea is reinforced in an uncomfortable scene where McPherson goes to Laura's apartment at night, reads through her private letters, looks through her closet and touches her clothes, eventually falling asleep in a chair under her portrait.