On a Friday afternoon in Phoenix, Arizona, Marion Crane and her lover, Sam Loomis, are having a romantic rendezvous at a hotel when Marion complains that she is tired of meeting Sam under such sordid circumstances.  Sam, who runs a hardware store in Fairvale, California, assures her that they can marry after he pays his debts, but Marion longs for immediate respectability.

Upon her return to the real estate office where she works as a secretary, Marion learns that her boss, George Lowery, is with oil tycoon Tom Cassidy.  When the men return, the lecherous Cassidy brags to Marion that he is paying $40,000 in cash to buy a house for his daughter.  Lowery, worried about leaving the money in the office over the weekend, tells Marion to take it to the bank, and Marion asks to go home afterward.

After rebuffing Cassidy again, Marion departs, but at her apartment, stuffs the money into her purse and leaves with a suitcase.  Driving until exhaustion forces her to pull over, Marion falls asleep on a lonely stretch of road.  She is awoken on Saturday morning by a highway patrolman, who is suspicious of her irritable manner.  After the policeman dismisses her, Marion, afraid that he will remember her, goes to a used car lot and trades in her vehicle for one with California plates.

Later, during a fierce rainstorm, Marion misses the turnoff to Fairvale and stops at the Bates Motel, where the proprietor, Norman Bates, welcomes her and offers to fix her dinner at his home, a looming structure on the hill behind the motel.  Marion accepts, but as she hides the cash in a newspaper she had purchased, she hears an old woman loudly berate Norman for attempting to bring a girl into her home.

When Norman returns with sandwiches, he explains to the apologetic Marion that his mother is ”not quite herself.” Norman then invites her into his parlor behind the office, where Marion is nonplussed by the birds Norman has stuffed in pursuit of his hobby, taxidermy.  Marion chats with the shy Norman, who confesses how alone he is, except for his mother.  When Marion asks if Norman has any friends, Norman replies that “a boy’s best friend is his mother,” although he admits that he wishes he could run away, as Marion is apparently doing.  Norman relates his belief that everyone is in a trap of some kind, and that his mother is mentally ill due to the deaths of his father and later, her lover.  When Marion suggests that Norman could lead a life of his own if he put his mother in an institution, he reacts bitterly, stating that his mother is harmless and that he could never abandon her.  Relaxing, Norman asserts that “we all go a little mad sometimes.”  Realizing that she has gone mad herself, Marion tells Norman that she has to return to Phoenix, in hopes of escaping a private trap.  Marion then goes to her room, unaware that Norman is watching her undress through a peephole.

While Marion writes a note calculating how much of the stolen money she has spent, Norman strides to the house, resolved to assert himself.  Norman’s strength fades, however, and as he sits dejectedly at the kitchen table, Marion tears up her note, flushes it down the toilet and enters the shower.  As Marion enjoys her shower, a shadowy female figure enters the bathroom and repeatedly stabs her.  A few minutes later, in the house, Norman screams out to his mother about the blood, then rushes to find Marion, lifeless on the bathroom floor.  Sickened but determined to protect his mother, Norman wraps Marion’s body in the shower curtain and after cleaning the room, deposits her corpse and belongings into the trunk of her car.  Norman also tosses in the newspaper, which he does not know holds the money, then sinks the car in a swamp behind the house.

A week later, as Sam is writing to Marion, he is interrupted by her sister Lila, whom he has never met.  Sam is baffled by Lila’s frantic questioning about Marion and is prevented from answering by the arrival of Milton Arbogast, a private investigator.  Arbogast and Lila explain to Sam about Marion’s theft, and although Sam maintains his innocence, Arbogast remains suspicious that he is involved.  Promising Lila that he will find her sister, Arbogast then spends two days searching the area.  When he reaches the Bates Motel, he interrogates Norman, who stammers that he has never seen Marion.  Arbogast uncovers Norman’s lie, however, and after Norman admits that Marion was at the motel, the detective appears to accept his statement that she left early in the morning.  When Arbogast sees Mrs.  Bates sitting in a window of the house, he wants to question her, but Norman orders him to leave.  Unsettled, Arbogast calls Lila and relates everything that Norman said, then states that he will return to Fairvale after interrogating Mrs.  Bates.  As Arbogast climbs the stairs in the house, however, he is stabbed to death by a woman.

Soon after, Norman sinks Arbogast’s car in the swamp, while in Fairvale, Lila grows impatient about the detective’s absence and Sam eventually takes her to see Deputy Sheriff Al Chambers.  Convinced that Arbogast got “a hot lead” from Norman, then left to chase Marion and the money, the skeptical Chambers dismisses Lila’s concerns, especially when she mentions Mrs. Bates.  Chambers explains that, ten years earlier, Norman’s mother poisoned her lover upon discovering that he was married, then committed suicide.  After Chambers telephones Norman, who confirms that Arbogast left suddenly, Norman confronts his mother, telling her that she must hide in the fruit cellar for her own protection.  Over her loud objections, Norman then carries her downstairs.

Unsatisfied by Chambers’ remarks, Lila and Sam drive to the motel the following day and check in.  After sneaking into the room in which Marion stayed, Lila finds a piece of the paper on which Marion had written.  Convinced that Norman hurt Marion to steal the money, Sam detains him in the office while Lila searches for Mrs.  Bates.  Norman, irritated by Sam’s insinuations, retreats to his parlor and upon hearing Sam’s mention of his mother, knocks Sam unconscious.

Meanwhile, Lila has been exploring the house, in which she finds Mrs.  Bates’s immaculate bedroom and her bed, which bears the imprint of her body.  Lila also snoops around Norman’s squalid room, which contains his childhood toys and a small cot.  Returning to the first floor, Lila sees Norman running up to the house and hides downstairs.  As Norman goes upstairs, Lila creeps down to the fruit cellar, where she finds Mrs.  Bates sitting with her back to the door.  Lila inches forward to tap the old woman on the shoulder, but when she swings around, Lila is horrified to find herself staring at a decaying corpse.  As she screams, Lila turns around to see Norman, wearing a wig and one of his mother’s dresses.  Shrieking “I am Norma Bates,” Norman lunges toward her with a knife, but Sam arrives in time to overpower him.

Later, as Sam and Lila wait with Chambers and other officials at the courthouse, Norman is examined by a psychiatrist, Dr.  Richmond.  Richmond explains that Norman, who suffers from a split personality, has been taken over by the dominant personality, that of his mother, and that Norman himself no longer exists.  Richmond states that after the death of his father, Norman was overwhelmed by his domineering mother, and that when she took a lover, Norman killed them both.  Unable to bear the guilt, Norman preserved her corpse, then, to heighten the illusion that “Mother” was alive, began dressing and speaking as her.  Believing that his mother would be as jealous of him as he was of her, Norman subconsciously allowed the Mother side of his personality to murder any woman whom he found attractive.  As they discuss the case, Norman sits in a nearby room, huddled in a blanket, while the Mother side of his personality thinks to herself that she could not allow her son to brand her a killer.  Noticing a fly on her hand, Mother cunningly declares that she will not swat it, so that anyone observing her will know that she would not even harm a fly.