On October 22, 1981, college students Terry, Nancy, and Gloria embark for a music festival in rural Washington. While driving on a forested road, the women smoke marijuana in the car, while a news radio broadcaster announces the recent disappearance of two teenage girls in the area. A sudden storm causes Nancy to accidentally crash, rendering all three unconscious.
Terry awakes to find her and her friends alive, sheltered in an isolated mansion owned by the Penroses: the middle-aged spinster Marion, her mother, and their groundskeeper, Norman. Gloria is the only one with serious injuries, so Marion suggests they spend the night until Gloria is able to leave. Terry and Nancy are invited to dinner with Marion and her elderly, senile, crippled mother. Throughout dinner, Marion's mother rants and raves, espousing her misandrist views, and recounts how her husband left her for another woman when the family lived as socialites in Rhode Island. She also recurrently accuses Marion of bringing men into the home. Later, while the women relax in the parlour, a mysterious man watches them through the windows.
Later that night, Terry finds a human tooth under her bed and subsequently awakens to the sound of a man breathing heavily upstairs. In the morning, Terry and Nancy take a shower, while someone spies on them through a peephole in the wall. Later that day, Nancy elects to walk to the local village alone and starts off through the woods. When she arrives at a rural country road, she is attacked by a cloaked figure with a long scythe, who slashes her to death. That evening at dinner, Mrs. Penrose continues to ramble about her disgust for men and harangues Marion, while Terry worries about Nancy's absence. Alone in her bedroom, Terry once again hears the breathing and goes to investigate. She searches the attic, where she finds black-and-white pictures of two children and an old tool belt containing a dusty handgun and a machete. She goes back downstairs and sees the man staring in at her through the window, and runs screaming through the house. Marion consoles her and reveals that the man is Carl, her developmentally disabled younger brother. Marion insists that he is harmless, and Terry goes back to bed.
The next day, Terry asks Norman if he has seen Nancy. Norman reveals that he never spoke with her, and instead tells her a confusing story about the two teenage girls who recently disappeared in the woods. At nightfall, Gloria regains consciousness, and Terry tells her she feels the two need to leave. When Terry leaves the room, an unseen figure attacks a sleeping Gloria, plunging a hatchet through her head. Later in the evening, Terry finds Gloria's room empty and asks Marion where she is. Marion suggests she may have gone outside for a breath of fresh air. As she steps outside, Terry is attacked and chased by Carl. She hides in a shed, where she discovers the dead bodies of her friends along with several other dismembered corpses. Carl breaks through the window and tries to grab her, but she manages to escape and runs back to the house as Carl chases after her.
Hurrying into the attic, Terry obtains the handgun and shoots Carl in the head, killing him. Marion rushes upstairs after hearing the struggle and chastises Terry for killing her brother. Terry responds by ordering Marion to search the shed. After a moment of silence, Marion, now speaking in a deep, masculine register, tells her that Carl had nothing to do with the corpses in the shed. Terry looks at Marion in confusion, who brandishes a machete. Marion reveals that she is actually Mrs. Penrose's second son, and Carl's younger biological brother, who dresses and presents as a woman. Terry attempts to flee, but Marion knocks her to the ground and maniacally stabs her to death whilst raving about the pressures of her gender dysphoria, and of her obligations to care for her brother and mother. As Terry bleeds to death on the floor, Mrs. Penrose calls for Marion from downstairs, asking if there is a man up there. Marion, covered in blood, responds in her feminine voice: "No, mother."
Terry discovers the bodies of her friends and several other victims in a shed after being chased by Carl, the Penrose family's developmentally disabled brother, revealing the true horror behind the mansion's façade. The film concludes with the uncovering of the killer's identity.
The ending cements Unhinged as a slasher that leans into both psychological and physical threats, drawing on twisted family dynamics and rural isolation to keep the audience on edge. The revelation of the bodies and the identity of the murderer reveal the deadly consequences of misplaced trust and the true peril hiding within the supposedly safe mansion. It's a final act that delivers both visceral horror and a grim sense of inevitability.
