After Marian Forbes is spurned by Harris Chapman, the wealthy Seattle businessman whom she has served as secretary, financial advisor and mistress for many years, she plots revenge and forms an alliance with a male colleague to impersonate Harris and deprive him of his identity, his money and his life. Harris has planned to spend several days in Mexico on a fishing trip, and Marian plans to use his absence to execute her plan.
In Mexico, Marian tutors her accomplice on Harris' vocal mannerisms and personal biography. On the night that Harris checks into his hotel, the desk clerk hands him a note from Marian, asking him to call her. When Harris phones, Marian threatens to expose irregularities in his 1955 income tax returns unless he meets with her. Outraged, Harris jumps into a cab and drives to Marian's secluded hideaway in the hills. There, Marian coldly announces that she she plans to replace him and then guns him down. Soon after, her accomplice arrives to don Harris' personal effects and assume his identity.
After wrapping the body in a blanket, the accomplice, who is now fully impersonating Harris, lugs it into the bathroom and dumps it in a tub. When he passes Marian, he finds her sprawled out on the bed, beckoning to him. Repulsed by her advances, he returns to the hotel and, after placing a "do not disturb" sign on Harris' door, meets Marian in the parking lot. He then drives Marian to the airport to catch a plane to Seattle, and returns to the hideout, where he attaches a trailer hitch and boat to the car. After loading the body into the boat, he drives to the coast, rows out to sea and dumps the corpse. His first test comes when, as Harris, he calls his fiancée Frances in Seattle.
After Frances tells him that she plans to visit friends in San Francisco during his absence, he goes to see Carreras, a Mexican businessman, to discuss developing a luxury resort in Mexico and offers to invest $250,000 in the project. Upon returning to his hotel, he receives a call from Harris' stockbroker Chris and irascibly orders him to sell $10,000 in stock and wire it to Mexico. At the hotel bar, a tourist mistakes Harris for a man he met on a fishing trip in Florida. After loudly asserting that his name is Harris Chapman, the imposter avoids the man by joining an attractive woman who is seated alone.
The woman introduces herself as Corey Scott and Harris invites her to join him shopping the next morning. A crisis arises later that night when an hysterical clerk from his Seattle office calls for the combination to his old safe, in which a pet dog has been trapped. As Harris tears through his wallet in search of the numbers, a telegraph arrives from Marian alerting him to the correct code, which he passes along to the flustered clerk. The next day, Harris returns from fishing to learn that Judge Kendall and his wife Edna, old family friends, have stopped by the hotel to visit.
After making arrangements to meet Corey for a drink later, Harris approaches the front desk to exchange some dollars for pesos. Just then, the Kendalls appear and he quickly pulls off thick horn-rimmed glasses that Harris wore and turns his back to them, hoping they will not notice his presence. Escaping detection, Harris phones Frances and tells her that Marian is pursuing him. After hanging up, he notifies the front desk that he is expecting a woman named Marian Forbes, and when Corey appears, the clerk assumes that she is Marian and sends her to Harris' room.
To avoid the Kendalls, Harris decides to alienate them, so when Corey arrives, he phones them up and makes sure they hear that there is a woman in his room, thus infuriating Edna, who thinks he is cheating on Frances. After dancing the night away with Corey, Harris invites her to his room and pretends to pass out, drunk. Corey pockets his wallet and slips away, and Harris smiles, hoping that she will take the money and leave town. In the morning, Carreras takes Harris to his bank, where he opens an account.
Harris then instructs Chris to wire $250,000 to the Mexican bank. In a series of phone calls. Harris regales Frances with elaborate plans for their European honeymoon, while in reality, he is dallying with a bevy of beautiful señoritas. Returning to the bank, Harris closes his account and piles the cash in his attache case. When Marian arrives at the hotel to split the profits, Harris encircles her neck with his hands and announces that he plans to kill her and plant her blackmail note next to the body, thus casting suspicion on Harris for the murder.
He continues that the charade with Corey was intended to make people believe that she was Marian, and was hounding Harris. Just then, Corey knocks at the door, accompanied by the police. Breaking into hysterical laughter, Marian informs him that Corey is really Harris' fiancée, Frances. Frances then explains that she came to Mexico to surprise Harris, and upon discovering that a strange man was impersonating him, contacted the police and has been working with them to expose him.
Marion Forbes is the secretary, the lover and the creator of the financial fortunes of Harry Chapman, but Chapman falls in love with Francis and decides to marry her. The revenge of Marion is terrible. With the help of the third voice she kills Harry who is then impersonated by the third voice. All of this to steal $600,000.
