Kim Novak plays Judy Barton, a vivacious young shop clerk who is approached by businessman Gavin Elster ([[Tom Helmore]]) with a startling proposition: Elster, who has by marriage acquired a sizeable fortune in the shipping industry, intends to kill his wife. Judy happens to resemble Madeleine Elster — the wife — and Elster offers to pay Judy a large sum of money if she will consent to impersonate Madeleine in order to provide Elster with an alibi. Judy accepts.
We are then introduced to "Scottie" Ferguson (Stewart), a retired [[police]] [[|police detective]] [[friend]] of Gavin Elster. Elster asks Scottie to spy on his wife Madeleine (who is really Judy in disguise), telling him that she has been acting strangely and he suspects she may be suicidal. Scottie trails Judy to an [[museum|art museum]], where he watches her as for three hours she stares intently at a portrait of Carlotta Valdes, a Spanish matron who has been dead for more than two centuries. Then she goes and buys flowers and puts them on Carlotta's grave.
In this and other ways, Judy gradually builds up a character before Scottie's eyes. He learns that "Madeleine" is depressed; that she is obsessed with a dead Spanish woman; and that she has only one suit, which is gray. After a few days of observation, Judy takes a daring risk: she leads Scottie to a ledge below the [[Golden Gate Bridge]] (a familiar backdrop in Hitchcock's films) and, without a look backward, jumps over the edge as if she were committing [[suicide]]. Scottie races to save her, and in this way she meets Scottie face to face.