A college student, Scotty Parker, finds off-campus housing after she arrives late for registration. She rents a room in a Victorian home owned by the Engels family overlooking the ocean. Another student, Peter Ranson, also rents a room at the same time. As Mason Engels shows Scotty her new accomodation and tells her the house rules, he reveals that the room once belonged to his sister, Victoria. Soon, Scotty knocks on Mason’s door and asks if she can store her suitcases in the attic. Mason directs her upstairs, but asks her not to disturb his mother, Mrs. Engels, in her room, located near the attic staircase. As Scotty leaves the attic, her landlord looks at her disapprovingly, and stares without uttering a word. Soon, another student-tenant named Jack Towne arrives, and is told by boarder, Doris Pritchard, that Peter is buying the roommates dinner. Later, at the restaurant on the beach, the group becomes acquainted over drinks. As they walk back along the water, Doris is determined to keep drunken Peter at arm’s length, while Jack walks Scotty to her room and kisses her. When the tide comes in, Doris takes off her shoes and scampers ahead, but Peter lingers behind on the beach, and she doubles back. Only seeing his jacket, she starts to panic when he sneaks up on her. Angrily, she runs away, and Peter passes out in the sand. The tide awakens him the next morning, and an unknown killer stabs him to death. Later, a jogger and her dog find Peter’s body buried in the sand. As Sgt. Manny Rusin observes Peter’s autopsy, he tells his partner, Lt. Sandy McGiver, that they found nine entry wounds on the body. Later, Manny interrogates Doris about the previous night. When Doris is upset by Manny’s questions, Scotty chimes in that Peter was drunk and rude, which was the reason Doris left him. Meanwhile, Sandy interrogates Mason, who did not see the tenants return, so Sandy finishes his questioning and leaves with Manny. After Mason tells his mother the police are gone, he returns to his room and locks the door. He opens his father’s army footlocker and talks to the man’s photograph, complaining about the pain the murder has brought, and how his mother is oblivious to his feelings. On campus, Jack convinces Scotty to take a swim with him on the beach near the house. After their swim, Scotty becomes uneasy and they leave. At the police station, Sandy tells the police chief that there is no weapon or witnesses to Peter’s murder. The chief asks Sandy to find concrete evidence that he can tell Peter’s parents. On the way back to the house, Scotty confesses that she is attracted to Jack, and they kiss. Back at the station, the officers discover that Mason’s sister, Victoria Engels, attacked her former boyfriend and his new girl friend with a knife at a party in the 1960s. They decide to search for Victoria. Scotty goes to the basement, and finds Doris doing laundry. Doris admits to feeling unsettled since the murder. She would like to talk more with Scotty, but Scotty goes upstairs to finish her homework. Jack surprises Scotty in her room, and they make love. Meanwhile, Doris hears the sounds of Scotty and Jack’s passion traveling through the ducts. While she listens, an anonymous killer repeatedly stabs her to death. Upstairs, Scotty kicks Jack out of bed on the pretext that she has schoolwork. In a secret bedroom located in the attic, Victoria admires a black and white photograph of her younger self. At one end of the room, she has propped Doris’ corpse in a chair and applied heavy makeup to her face. Victoria peers into Doris’ lifeless eyes and motions her to be quiet. Next, Victoria looks through the keyhole, and sees her mother asleep in a chair near her door. Later, Scotty goes to the basement, and looks for Doris. She discovers a hidden set of stairs and searches the passageway. At a doorway, she examines a bed sheet stained with blood, and Victoria grabs her. As Scotty wrestles free, Mrs. Engels wakes up and tries to separate them. Mason hears the commotion and runs to the attic. After Jack removes his earphones, he, too, hears noise, and heads to the attic after seeing Scotty is not in her room. Jack tries Victoria’s door, but Mrs. Engels and Mason have barricaded it and clamped Scotty’s mouth shut. Jack leaves to search for Scotty and Doris. Upstairs, Mason ties up Scotty in Victoria’s closet, and complains that his mother has ruined his life. All she cares about is protecting Victoria. Mrs. Engels tells him that Victoria was pregnant with Mason when she tried to hang herself long ago. Both she and the baby survived, and Mrs. Engels brought her home instead of committing her to a mental institution. Although she told Mason that his father was a military officer, the man Mason believes is his father is really his grandfather. Mason refuses to believe the truth and locks himself in his room. When Jack asks about Scotty, Mason knocks him unconscious. Mason opens his grandfather’s footlocker and takes out his gun. At the police station, Manny reports that Victoria was institutionalized for three years after her suicide attempt, which followed the knife incident at the party. Although Victoria was lobotomized for her increasingly violent behavior, she left the hospital fifteen years earlier with her mother. The officers then race over to the Engels residence. Meanwhile, Mason is dressed in his grandfather’s army uniform, and has loaded the pistol. He returns to Victoria’s room, and says there is only one way they can all escape. Mrs. Engels struggles to free the gun from Mason, but she is shot and killed. As Mason strokes Mrs. Engels’ body, Victoria pulls a large knife from her mattress and approaches Scotty. However, Victoria changes her mind and heads toward Mason with his back turned. Scotty warns Mason by making guttural sounds through her gag, and he shoots Victoria. Then, Mason turns the gun on himself. Scotty breaks free from one of her restraints just in time to slam the closet door before Victoria attacks her. Jack hears the door slam and finds Victoria. Meanwhile, the police arrive at the house. Before Victoria can decide whom to attack first, Scotty pushes Victoria into the wall, and the knife impales her. The police hear the noise and rush to the attic. There, they see the dead bodies of Mrs. Engels, Mason, and Victoria. A trail of blood leads them to Jack, comforting a tearful Scotty.