In a Phoenix hotel room on a Friday afternoon, Marion Crane (Janet Leigh) and her out-of-town lover Sam Loomis (John Gavin) end a stolen lunchtime interlude with yet another disagreement about their future.
Marion wants to marry Sam, but debts inherited from his father and his own alimony payments do not leave him enough money to support her as he would like.
As they have done so often before on Sam's business trips to Phoenix, they part leaving their future uncertain.
Marion returns to the real estate office where she works as a secretary, arriving just ahead of her boss Mr. Lowery (Vaughn Taylor) and his client Cassidy (Frank Albertson) who buys a house from Lowery with $40,000 in cash.
Lowery tells Marion to put the money in the safe deposit box at the bank until Monday. Pleading a headache, Marion asks to take the rest of the day off after her errand to the bank.
But Marion doesn't go to the bank. On the spur of the moment, she decides to keep the money, packs a suitcase, and starts driving out of town, only to be spotted by her boss at an intersection where he gives her a suspicious look.
Worried that she has been found out already, she still proceeds out of town on her way to Fairvale, California, where Sam lives. All the while she keeps looking behind her, fearful that she's being followed. She drives well into the night and parks alongside the road to sleep.
In the morning, a highway patrolman (Mort Mills) stops to investigate her stopped car, and awakens her.
Startled and nervous, she arouses the patrolman's suspicions. He looks at her license and registration, taking note of the plate number.
He allows her to go on, but follows her for a while, which intensifies Marion's agitation. Realizing that her car can easily give her away, Marion decides to trade it in for a different car.
She stops in at a used car lot, hurriedly pays the salesman (John Anderson) $700 cash for a likely substitute, and completes the deal.
The same highway patrolman watches from across the street. Nervous, she drives away and continues toward Fairvale.