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Norman Bates (Anthony Perkins) is released from a mental institution after spending 22 years in confinement. Lila Loomis (Vera Miles), sister of Marion Crane, vehemently protests with a petition that she has been circulating with signatures of 743 people, including the relatives of the seven people Norman killed prior to his incarceration, but her plea is dismissed. Norman is taken to his old home behind the Bates Motel by Dr. Bill Raymond (Robert Loggia), who assures him everything will be fine.
Norman is introduced to the motel's new manager, Warren Toomey (Dennis Franz). The following day, Norman reports to a prearranged job as a dishwasher and busboy at a nearby diner, run by a kindly old lady named Emma Spool (Claudia Bryar). One of his co-workers there is Mary Samuels (Meg Tilly), a young waitress. After work, Mary claims she has been thrown out of her boyfriend's place and needs a place to stay. Norman offers to let her stay at the motel, then extends the offer to his home when he discovers that Toomey has turned what had been a shabby but respectable motel into a sleazy adult one.
Norman's adjustment back into society appears to be going along well until "Mother" begins to make her presence known. Norman gets mysterious notes from "Mother" at the house and diner. Phone calls come from someone claiming to be Norman's mother. The next day, a drunk Toomey picks a fight at the diner after Norman fires him. Later, a figure in a black dress stabs Toomey to death with a kitchen knife as he is packing to leave the motel. As Norman begins to reconstruct his motel, he begins to doubt his sanity when he begins hearing voices in the house. He enters his mother's bedroom to find it looks exactly as it did 22 years ago. A sound lures him to the attic, where he is locked in.
At the same time, a teenage couple, believing the house to be abandoned, sneaks in through the cellar window. They notice a female figure pacing in the next room. As they try to climb out, the boy is stabbed to death. The girl escapes and alerts the police. Mary eventually finds Norman in the attic. Minutes later, the sheriff arrives and questions them about the boy's murder. He finds the cellar neat and orderly. Norman is about to admit that something suspicious is going on, but Mary claims that she has cleaned up the basement herself. After the sheriff leaves, Norman asks Mary why she lied. She explains that she had to save him from being arrested. Norman collapses into the chair with his head in his hands and moans, "It's starting again!" Norman is aware that he is slipping into insanity again.
That evening, Mary is startled when she discovers someone looking at her through a peephole in the bathroom wall. She calls out to Norman, who is downstairs and out of reach. The two are horrified to find a bloody cloth that has been stuffed down the toilet. Norman appears confused and believes he may have committed another murder. Mary goes down to check the motel. In the parlor she is surprised by Lila, who reveals herself to be Mary's mother. She has been calling Norman, claiming to be his mother, even going so far as to dress up as her and allowing him to see her in the window. Mary has been helping her. She was responsible for restoring Mother's room at the house and locking Norman in the attic. All of this was an attempt to drive Norman insane again and have him recommitted. Mary's growing feelings for Norman, however, have been preying on her conscience leaving her to reconsider her actions. Meanwhile, Dr. Raymond discovers Mary's identity as Lila's daughter and informs Norman. He also orders the corpse of Norma Bates (which was buried in a proper grave after the events of the original film) to be exhumed, to prove that Norman is not being haunted by his mother. Mary admits to Norman that she has been part of Lila's ruse, and that while she now refuses to continue, Lila will not stop. Mary goes to Lila's hotel and their argument is overheard by a desk clerk. Later, Lila drives over to Norman's house, unaware that Dr. Raymond is watching her from the Bates Motel as she sneaks into the cellar.
While Lila is removing her "Mother" costume from a loose stone in the floor, another figure dressed as "Mother" steps out of the shadows and murders her. Dr. Raymond runs up to the house. Lila's body is not in the cellar and the "Mother" figure is gone. Meanwhile, Mary discovers that a car has been retrieved from the swamp, with Toomey's body in the trunk. Realizing the police will shortly arrive to arrest Norman, Mary returns to warn him. The phone rings in the house, Norman answers, and starts speaking to his "Mother". Mary listens in and discovers that nobody is on the line with Norman. Terrified that Norman has slipped back into insanity, Mary runs downstairs into the cellar and quickly dresses up as Mother to confront Norman and arms herself with a butcher knife. Someone grabs her from behind as she is on the stairs and plunges the butcher knife into Dr. Raymond's heart, who has sneaked back into the house, killing him instantly. A stunned Mary runs downstairs and is confronted by a completely deranged Norman, who promises to cover up for "Mother." Mary tries to keep him away, but she repeatedly keeps stabbing him in the hands and chest, none of which stop Norman. He backs Mary into the fruit cellar to hide and slips on a pile of coal, which avalanches away from the wall, revealing Lila's dead body hidden behind it. Mary is now convinced that Norman has been committing the murders. Norman denies doing any of the killings and thinks "Mother" committed them. She raises her knife to stab him but is shot dead by the incoming police deputy. The sheriff inaccurately believes Mary committed all the murders.
That evening, a woman walks up the steps to the Bates' mansion. Bandaged from his injuries, Norman has set a place for dinner when he hears a knock at the door. It is Emma Spool, the kindly woman from the diner. Norman gives her a cup of tea, brewed from a poisoned batch which was kept in a beaten floral-print tin; this was the tea he used to kill Mrs. Bates, his "Mother," when he was a child. Ms. Spool tells him that she is his real mother, that Mrs. Bates was her sister, who adopted Norman as an infant while Ms. Spool was institutionalized. She further reveals that she was the murderer, having killed anybody who tried to harm her son. As she begins dying after sipping the tea, Norman strikes her with a sudden blow to the head with a shovel, now driven completely insane again. He carries Ms. Spool's body upstairs to Mother's room and we hear Mother's voice warn Norman not to play with "filthy girls" and telling him nobody loves him like his mother. Norman reopens the Bates Motel and stands in front of the house, waiting for new customers as Mother watches from the window upstairs.
Who is Lila?
Mary's Mother