In Phoenix, Arizona, lovers Marion Crane (Leigh) and Sam Loomis (John Gavin) want to marry, but cannot, as Sam is in debt and must also pay heavy alimony to his ex-wife. Unhappy and desperate to improve their situation, Marion steals $40,000 in cash from her office and drives to California, where Sam lives. All the while, Marion is nervous and apprehensive, and drives well into the night, eventually parking alongside the road to sleep. She is awakened by a concerned highway police officer, who warns her that it is dangerous to sleep in a car and tells her in the future to find a motel. However, Marion’s desperation to leave arouses his suspicions. He allows her to go on, but follows her, which agitates Marion further. Realizing that he now knows her plate number, she trades her 1956 Ford Mainline for a 1957 Ford Custom 300 before continuing to California. However, the same officer has been watching the exchange from across the street and gotten her new plate number. Marion leaves, worrying that the car trader will express suspicions of his own to the officer.
Marion becomes fatigued from stress and driving in heavy rain and decides to find a proper place to stay for the night, fearing a reprise of the incident with the patrolman. She turns off the main road without realizing it, and arrives at the Bates Motel, a 12-cabin lodging, rather out-of-the-way with no other guests at present. The owner, Norman Bates (Perkins), explains to her that business has decreased dramatically since the new road bypassed the motel. Norman does what little work is left, and also looks after his mother in a sinister-looking house on top of a nearby hill. Marion checks in under an assumed name, though she unwittingly gives her real name to him later.
It is still raining, Marion is very tired, and the nearest diner is ten miles away, so Norman suggests that she have dinner at his house. However, from her room, Marion overhears a heated argument between Norman and his mother, who seems to suspect that his meal with Marion is part of a sordid affair. The two eat in the office instead, where Norman keeps several stuffed birds (his hobby is taxidermy). While eating, they have a gentle conversation at first, but Norman becomes angry after she delicately suggests he institutionalize his mother. Norman recovers from his brief outburst and admits that he would like to leave, but can’t abandon his mother. He compares his life to a trap and observes that this aptly describes most people. Feeling that the theft of the money has also got her into a trap, Marion resolves to drive back to Phoenix in the morning. She undresses in her room next door while Norman watches through a peephole in the wall of his office.
Resolving to make amends to her employer, Marion makes a few calculations based on how much the escapade has cost her. She then takes a shower. Suddenly, a human figure enters the bathroom — shadowy through the shower curtain — and brutally stabs Marion to death. She dies shortly after grabbing the curtain, which collapses. Back at the house, Norman yells to his mother that he notices blood, and runs to the motel to check on Marion. He is horrified when he finds the bloody corpse in the bathroom, but he pulls himself together and wraps it in the shower curtain. He cleans up the bathroom, then places the body and all of Marion’s possessions (including the stolen money which is still hidden in a newspaper) into the trunk of her car before pushing it into a swamp, eliminating any incriminating evidence.
Shortly afterward, Marion’s lover Sam is contacted almost simultaneously by Marion’s worried sister Lila (Vera Miles) and by a private detective, Milton Arbogast (Martin Balsam), hired by Marion’s employer to find her and recover the money himself, as her employer does not want police involvement. Arbogast suspects that either of them could know Marion’s whereabouts. However, he traces the missing woman to the Bates Motel and questions Norman. At this point, Norman suspects his mother of killing Marion, and to protect her, he lies poorly, stuttering while speaking. Arbogast wants to speak to Norman’s mother but the young man vehemently forbids it, saying she’s confined to bed and is too ill to see anyone. Arbogast then calls Marion’s sister from a public phone, and tells her that he is not satisfied with what he has been told and is going back to the motel. Before hanging up, he says for Lila to expect him back in an hour, or less. He returns to the Bates property, and when he doesn’t find Norman, he sneaks into the old house to question Mrs. Bates, but again, a human figure runs out of the bedroom, pushes him backwards down the stairs and stabs him to death.
Lila and Sam become concerned when Arbogast does not report again and decide to alert the local police. Deputy Sheriff Al Chambers (John McIntire) is puzzled that Arbogast has claimed to have seen Norman’s mother, and tells them that she has been dead and buried for the past 10 years, having (apparently) poisoned herself and her lover with strychnine. However, Sam insists that someone else lives in that house, saying that Arbogast said so and that he himself saw a woman sitting up in the window. The sheriff then begins to question whether or not that woman could really be Mrs. Bates. Meanwhile, the Bates’ house resonates with a conversation as Norman confronts his mother, urging her to go into hiding in the fruit cellar, as people are already searching for Marion and will eventually search for Arbogast as well. She rejects the suggestion, angrily mentioning a previous occasion when Norman convinced her to stay down there for a long time. She then orders Norman to leave the room, having to repeat herself, with each request growing more angry. He refuses, picks her up against her will and carries her downstairs to the fruit cellar, with her yelling “Put me down! I can walk on my own!”
Sam and Lila decide to check into the Bates Motel, posing as a married couple. Norman assigns them to a cabin away from Marion’s room. They sneak in anyway to investigate, and find that the shower curtain is missing. Lila looks into the toilet and sees a small scrap of paper caught at the edge. The sum of $40,000 is written on it, confirming that Marion had been there. Lila then sneaks into the house with the intention of talking to Norman’s mother, while Sam distracts Norman. Their conversation is casual at first, but Sam is convinced that Norman is somehow behind Marion’s murder, and voices begin to raise. He finally suggests to Norman that he has killed Marion to get his hands on her stolen money, with the intention of moving away and buying a new motel. They argue until Norman realizes that Lila is not present. Furious and panicked, he knocks Sam unconscious and races to the house. Seeing him come through a window, Lila hides from him in the fruit cellar, where she discovers Mrs. Bates sitting in a rocking chair, her back to her. After tapping her on the shoulder, she discovers the woman is a semi-preserved mummified corpse. Screaming in shock, she flings her arm, knocking the hanging light bulb. At that moment, Norman, wearing his mother’s clothes and a wig, enters, yelling and holding a knife. However, Sam has regained consciousness and arrives just in time to save Lila. He rips Norman’s wig and dress during their struggle.
A forensic psychiatrist, Dr. Fred Richmond (Simon Oakland), explains to Lila, Sam, and the authorities that Bates’s mother, though dead, lives on in Norman’s psyche. Dr. Richmond explains that while growing up, Norman lived alone with his mother, as if they were the only two people in the world. (”A boy’s best friend is his mother,” Norman had told Marion early in the film.) He grew very disturbed after his father died, and when his mother found a lover, Norman became jealous and murdered them both. He was so dominated by his mother while she lived, and so guilt-ridden for murdering her, that he tried to “erase the crime” by bringing his mother back to life in his own mind. Physically, this was done by stealing her corpse (”a weighted coffin was buried,” according to Richmond) and preserving his mother’s body using his taxidermy skills. This process also created a dual personality in Norman; he incorporated the persona of his mother as a separate part of his psyche. When he is being his “Mother”, he acts as he believes she would, talks as she would, and even dresses as she would. Because Norman was very jealous of his mother while she lived, he imagined that Mother would be equally jealous of any woman to whom he might be attracted, to the point of murdering them. Norman’s psychosis protects him from (consciously) knowing about the crimes the mother figure commits, and it also prevents him from consciously knowing that his mother is long dead. Besides Marion and Arbogast, the sheriff mentions the unsolved disappearances of two young girls.
The last scene shows Norman Bates seated in a cell. His mind is now completely dominated by the persona of his mother. We hear “her” internal voice as a voice-over. She blames Norman, and plans on demonstrating to the authorities that it was Norman who did the crimes, whereas she is utterly harmless. She knows that people must be observing her, and will show them what kind of a person she is. As a fly crawls on Norman’s hand, Mother continues, “I’m not even going to swat that fly. I hope they are watching. They’ll see, they’ll know, and they’ll say, ‘Why, she wouldn’t even harm a fly’”. We see “Mother” smile with satisfaction, which shows through Norman’s demented stare (a double exposure shot of Norman’s face over a bleached skull). The film’s final shot is of Marion’s car being recovered from the swamp.
