In a bleak, decaying Los Angeles neighborhood, young Poindexter "Fool" Williams faces imminent eviction by his ruthless landlords, the bizarre and oppressive couple known only as "Man" and "Woman." Driven by desperation to pay his mother’s overdue medical bills, Fool agrees to participate in a late-night burglary of the landlords' heavily fortified, labyrinthine Victorian mansion, orchestrated by his sister’s boyfriend, Leroy.Upon breaking inside, the heist quickly unravels. Leroy is brutally murdered by Man, leaving Fool trapped alone in the sprawling, nightmarish estate. As Fool navigates the twisting corridors, he discovers the horrifying truth behind the fortress: the couple are actually deranged brother-and-sister incestuous serial killers who abduct, torture, and mutilate children, keeping the survivors locked away in the pitch-black basement as literal prisoners and slaves.While hiding from the sadistic Man—who patrols the halls with a shotgun and a pack of vicious dogs—Fool befriends Alice, a terrified young girl who has spent her entire life locked away upstairs. Alice reveals the extent of the horrors committed by the incestuous pair, who refer to themselves as "Mommy" and "Daddy," and collect human flesh. Together, Fool and Alice attempt to navigate
the treacherous estate to escape. Meanwhile, Fool also encounters the feral, cannibalistic older children living in the crawlspaces beneath the floorboards—the "people under the stairs"—who were driven mad by years of abuse.As the night escalates into chaotic violence, the deranged landlords realize intruders are present and initiate a sadistic hunt. Man turns his wrath inward, increasingly blaming and punishing Woman for the
security breach. Amidst the chaos, Fool manages to free the captive feral children from the basement, inciting a violent reckoning against their tormentors. The basement dwellers rise up, overwhelming the abusive Man and dragging him down into the darkness to exact bloody vengeance.Simultaneously, Woman becomes unhinged, detonating explosives hidden within the walls in a final, psychotic attempt to destroy the house and everyone in it. Fool and Alice narrowly escape the collapsing inferno, carrying bags of the landlords' stolen money intended to save Fool's family. As the corrupted mansion goes up in flames, the surviving children emerge blinking into the morning sunlight, finally liberated from their lifelong nightmare, while Fool secures a brighter future for his mother and sister.