Psycho is one of the most twisty, sneaky film plots ever snuck and twisted onto a movie screen. If you haven't seen it yet, don't read this first.
Seriously, go watch the film, get surprised by the surprises, then come back. We'll wait.
Okay? You saw it, right? No cheating? All right, then: onward to gory death.
The film opens with Marion Crane. She's a real estate secretary in Phoenix, Arizona, but when the film opens, she's not at work. She's in a hotel having some quick afternoon delight (skyrockets in flight) with her boyfriend, Sam Loomis. Sam loves her and she loves him, but they can't marry because Sam has lots of debts, including alimony for his ex-wife.
Basically, Marion needs money to bail Sam out of debt so he can marry her. (This is a big plot point: remember this guy.)
Marion goes back to the office, where her boss comes in with a client. That client drops off $40,000 in cash (which is about $400,000 in today's dollars). The boss tells Marion to deposit the dough in the bank… but instead Marion takes the money and runs to Fairvale, California to give it to Sam.
Marion isn't a very good thief, because she's super nervous. She sees her boss passing in front of her car on the way out of town, which freaks her out. Then she gets tired driving and pulls over by the side of the road, which makes a cop come by—and she acts weird and panicked. She eventually ditches the cop and buys a new car. The cop sees her do that, too—oof. Marion, you are never going to be a member of Danny Ocean's eleven (or twelve, or thirteen).
Anyway, Marion drives on and then a rainstorm forces her off the road to an out-of-the-way motel. It's run by a shy young dude named Norman Bates, who lives in a gothic-looking house with his mother. Norman is friendly and seems attracted to Marion, but his mother is jealous and mean; Marion hears her yelling at Norman.
Pro-tip: if you ever see a hotel like this, run don't walk. And definitely don't take a shower.
Marion and Norman have dinner together in the office, and Norman explains that his mother, Norma, is mentally ill. Norman seems a little, um, off himself. But talking to him leads Marion to realize she can't go on being a criminal; she decides to drive back to Phoenix and return the money.
After dinner, Marion goes to her cabin, undresses, and gets ready to take a shower. Norman watches her from a peephole in the office. Norman is creepy. Important plot point.
Marion gets in the shower and a woman with a butcher knife suddenly appears and… stabs her to death. Main character: dead.
Norman is now horrified at what his mother has done. He runs to the cabin, cleans everything up, and then puts Marion's body and all her effects, including the stolen money, in her car, and sinks it in the swamp near the motel.
And hey: since the first main character is dead, you need a new one. Marion's sister Lila shows up at Sam's workplace in Fairvale and asks if he knows where Marion is. He doesn't. Then a detective, Abrogast, shows up. He's working for the client whose money Marion stole. But he believes Lila and Sam when they say they don't know where Marion is. He agrees to keep in touch with them.
Abrogast tracks Marion to the Bates motel. He interrogates Norman, and wants to talk to Norman's mother, but Norman won't let him. Abrogast calls Lila and Sam and tells them of his progress, and says he's going to go back to the Motel and try to interview Norman.
So back he goes, into the house, up the stairs, and yep, that same woman with the knife appears and stabs him to death. Second protagonist: dead. The movie is running out of protagonists.
Lila and Sam are freaked out when Abrogast doesn't call back. They go to the Sheriff, who talks to Norman on the phone, but doesn't think anything is wrong. He also tells them that Norman's mother has been dead ten years; she murdered her boyfriend and then poisoned herself.
So who is that with the knife?
Whoever it is, after Norman gets the phone call, he goes to his mother, picks her up, and takes her to the fruit cellar, to avoid anyone finding her. Mama Bates is none too happy: she's berating Norman the whole times.
Lila and Sam go out to the Bates Motel themselves, posing as man and wife. Lila is determined to talk to Norman's mother. Sam distracts Norman, and Lila goes into the house. However, Sam makes Norman suspicious, so Norman clubs him in the head, and rushes back home. His arrival causes Lila to hide in the fruit cellar, where she discovers Mrs. Bates—or rather, Mrs. Bates' corpse.
She screams (as who wouldn't?) and the woman with the knife rushes in. Before she can kill Lila, though, Sam appears, and grabs the stab-happy woman—only to reveal it isn't a her, but a him. The murderer was Norman, dressed up as his mother.
The final scene goes down in the police station, where a prison psychiatrist explains that Norman had killed his own mother out of jealousy of her new boyfriend. He had then exhumed her corpse and kept it in the house. Norman then began to believe he was his mother.
When he became interested in another woman, "mother" would get jealous, and Norman-as-mother would kill. The psychiatrist suggests that Norman has killed many women. He also says that Norman's personality has been destroyed, and only mother is left. (How does this dude know everything? He's a psychiatrist. Psychiatrists know all. Also, Norman, as his mother, explained everything.)
The film ends with Norman sitting in the police station with his mother's voice inside his head, insisting that it was Norman and not her who committed the murders. Norman looks absolutely bonkers at this point. He grins up at the camera, and you see a flicker of his mother's skull superimposed over his face.
Then the shot ends and we see Marion's car being winched up from the swamp. Mud is sliding off of its trunk to reveal its license plate in a way that not so subtly mirrors the previous shot of Mama Bates' toothy skull-mouth revealed beneath Norma's placid face.
Sweet dreams, everybody.