During a lunchtime tryst in a Phoenix, Arizona hotel, real-estate secretary Marion Crane and her boyfriend, Sam Loomis, discuss how they cannot afford to get married because of Sam’s debts. After lunch, Marion returns to work, where a client leaves a $40,000 (equivalent to $351,000 in 2019) cash payment on a property. Marion’s boss asks her to deposit the money in the bank and allows her to leave work early after she complains of a headache. Once home, she decides to steal the money and drive to Fairvale, California, where Sam lives.

En route to Fairvale, Marion stops her car on the side of the road and falls asleep. She is awakened the next morning by a state patrol trooper who follows Marion after questioning her and growing suspicious about her skittish behavior. Marion stops at a Bakersfield, California automobile dealership and trades in her Ford Mainline, with its Arizona license plates, for a Ford Custom 300 with California tags. The officer spots Marion at the car dealership and eyes her suspiciously as she abruptly drives away.

During a heavy rainstorm, Marion stops for the night at the Bates Motel. The proprietor, Norman Bates, invites her to share a light dinner after she checks in. She accepts his invitation but overhears an argument between Norman and his mother about bringing a woman into their Gothic home, which sits perched above the motel. Instead they eat in the motel parlor, where he tells her about his life with his mother, who is mentally ill and forbids him to have a life apart from her.

Moved by Norman’s story, Marion decides to drive back to Phoenix in the morning to return the stolen money, which she hides in a folded newspaper on the nightstand. As she showers, a shadowy figure stabs her to death. After seeing blood, Norman panics and runs to Marion’s room, where he discovers her body. He cleans up the crime scene, putting Marion’s corpse and her possessions – including (unbeknownst to him) the stolen money – into the trunk of her car and sinking it in the swamps near the motel.

A week later, Marion’s sister Lila arrives in Fairvale and confronts Sam about Marion’s whereabouts. Private investigator Milton Arbogast approaches them and confirms that Marion is wanted for stealing the $40,000. Arbogast sleuths local motels and discovers Marion spent a night at the Bates Motel. He questions Norman, whose stammering, skittishness and inconsistent answers arouse his suspicion. After Norman implies that Marion met his mother, Arbogast asks to speak with her, but Norman refuses to allow it. Arbogast updates Sam and Lila about his search for Marion and promises to phone again soon. He goes to the Bates’ home in search of Norman’s mother; as he reaches the top of the stairs, a mysterious figure stabs him to death.

When Lila and Sam do not hear from Arbogast, Sam visits the motel. Sam sees a figure in the house who he assumes is Mrs. Bates, but she ignores his knocking. Lila and Sam visit the local deputy sheriff, who informs them that Mrs. Bates died in a murder-suicide ten years ago. The sheriff concludes that Arbogast lied to Sam and Lila so he could pursue Marion and the money. Convinced that some ill has befallen Arbogast, Lila and Sam drive to the motel. Norman carries his mother from her room and hides her in the fruit cellar.

At the motel, Sam distracts Norman by engaging in conversation while Lila cases the property and sneaks inside the house. After Sam grills him, Norman becomes agitated, knocks Sam out, and rushes to the house. Lila hides in the cellar, where she finds Mrs. Bates in a chair. Lila turns her around and discovers she is a mummified corpse. Lila screams as Norman runs into the cellar, holding a chef’s knife and wearing his mother’s clothes and a wig. Before Norman can attack Lila, Sam – having regained consciousness – subdues him.

At the courthouse, a psychiatrist explains that Norman murdered Mrs. Bates and her lover ten years ago out of jealousy. Unable to bear the guilt, he stole her corpse and began to treat it as if she were still alive. He recreated his mother in his own mind as an alternate personality, dressing in her clothes and talking to himself in her voice. This “Mother” personality is as jealous and possessive as Mrs. Bates was while alive: whenever Norman feels attracted to a woman, “Mother” kills her. As “Mother”, Norman killed two young girls before stabbing Marion and Arbogast to death. The psychiatrist says the “Mother” personality has taken permanent hold of Norman’s mind. While Norman sits in a holding cell, “Mother”‘s voice-over protests that the murders were Norman’s doing. Marion’s car is towed from the swamp.