On a routine night, Don Drake and his brother Fred Horton visit the house of Doug Drake, Don's brother. They are unaware that Doug and his wife, Susan Drake, who have struggled to conceive naturally, have been involved in unconventional experiments led by Dr. Lucas to artificially induce pregnancy. While they search for beer, they stumble upon a book written by Aleister Crowley and a tape recorder that plays a distorted voice. The eerie recording unsettles the group, and after hearing it, Doug storms in, scolds them, and then reluctantly joins them in their drinking.
Susan’s desperate longing for a child takes a dark turn as she gives birth to insect-like creatures before dying. The newborn beings quickly attack and kill a dog, setting a chilling tone for what follows. Doug explains the nature of the experiments he and Susan had pursued with Doctor Lucas, insisting that things have gone terribly wrong. The power in the house suddenly dies, and the creatures born from Susan begin to infest the dwelling, turning the night into a perilous fight.
Don and Doug search the house with a flashlight, moving cautiously through the dim rooms. In the basement, a creature attacks Doug from behind. Don raises a hammer to strike the threat, but in a tragic misstep, he accidentally bludgeons Doug instead. He drags Doug back upstairs, where another creature bites off several of Doug's fingers. Don works to cauterize the wounds, but Doug dies soon after. To keep the rest of him safe from the advancing creatures, Don hides Doug’s body in a closet.
Armed with a drill, Don continues to roam the house, haunted by visions of Doug still being alive. He falls asleep on a couch, and soon Fred returns with a chainsaw. Don finds a golf club and together they fight the invading creatures, killing several of them. They uncover a horrifying scene: the creatures have eaten Susan’s body, leaving only bones. The danger intensifies as the creatures turn on Fred, who is eaten alive while he pleads for Don’s help.
Doctor Lucas arrives at the house, and Don openly accuses him of bearing responsibility for the cascade of deaths. The creatures ambush Lucas, and Don manages to escape the house and encounter a passerby who walks with him toward a car. He asks, > Are you sure this wasn't all a dream? Afterward, he is shown back in the house, standing beside a bloodied but apparently living Doctor Lucas.
