Glen, portrayed by Stephen Dorff, arrives home only to discover it abandoned, with no one responding to his calls. The kitchen reveals a half-eaten dinner, while laughter echoing nearby adds an eerie feel. Venturing into the backyard, he climbs up into a treehouse, where he finds a lit lantern and a doll. Suddenly, his world turns upside down as lightning strikes the tree, causing it to fall.
Awakening to the sounds of construction work, Glen realizes it was merely a nightmare. He looks out the window to see workers removing the tree and unearthing a strange fragment of a geode. Determined to explore further, Glen returns with his friend Terry to dig at the site. Despite the workers' attempts to close the hole, the two friends unearth a large geode, though Glen suffers a splinter, leaving a trail of blood behind. With their parents away for three days, Glen's older sister Alexandra, affectionately known as "Al," takes charge and decides to throw a party.
As the festivities continue, Glen and Terry crack open the geode, revealing bizarre inscriptions left on a notepad. Curious, they read the incantations aloud just as the party guests begin a levitation game, astonishingly managing to lift Glen into the air. That night, Glen experiences a surreal event where his bedroom wall appears to stretch, while Terry unexpectedly embraces the ghostly image of his deceased mother, only to find it is the body of Angus, Glen's beloved dog.
The next day, Terry arrives with a heavy metal album featuring lyrics inspired by something ominously referred to as The Dark Book. He suggests that the hole in Glen's backyard could be a portal to malevolent gods, speculating that their previous activities might have awakened something sinister. In a tragic twist, Al's friend unwittingly completes the summoning by discarding Angus's body into the hole. While the boys believe they have closed the gateway by reading from The Dark Book, they are unaware of the evil that has already been unleashed.
That night, a swarm of moths burst through Glen's window, and Terry discovers Angus's corpse in his bed. Al is nearly pulled under her bed by demonic hands, prompting a frantic rescue from Glen and Terry. Attempting to escape, they encounter their parents but quickly realize that the figures are actually disguised demons. In a bid to regain control, Al volunteers to check the yard but is called back upon seeing swarming little demons.
Desperate for answers, the group goes to the basement to retrieve The Dark Book, but it suddenly ignites into flames, leaving them feeling hopeless. They turn to the Bible to combat the forces they inadvertently summoned. Terry reads from Psalm 59, and the hole begins to close, yet in a moment of turmoil, he falls in and is attacked by demons. Al and Glen pull him out just as the hole starts to seal, and amidst the chaos, Terry throws the Bible into the hole, leading to an explosion that knocks them unconscious.
Upon awakening, they find the hole sealed, but not for long, as a wall breaks open, and a body tumbles through. Glen wonders if this could be the construction worker rumored to have been buried within the walls. Before they can process this, the construction worker grasps Terry, pulling him into the wall, sealing it behind him.
Al senses something sinister when she spots a hazy figure of the construction worker in her mirror. Glen rushes into Al's room, and the ominous figure confronts them. In a frantic attempt to fend him off, Al throws a stereo, causing him to dissolve into numerous little demons. An intense standoff ensues as Al, holding the bedroom door shut, sees Glen racing downstairs for their father's gun.
A nightmarish version of Terry then attacks Glen, forcing Al to intervene by stabbing him in the eye with a Barbie doll's foot. Hiding in a closet, they witness the construction worker break through a wall, and in an act of desperation, Al fires a shot at him, but he remains unharmed, snatching her away.
Realizing that Al and Terry represent the two sacrifices needed to fully open the gate, Glen understands that the rocket Al intended to gift him, symbolizing love and innocence, is key to stopping the rise of the dark entities. Just then, the floor collapses, exposing a chasm beneath the house. Glen retrieves the rocket but struggles to ignite it as the winds intensify. As he is sucked into the foyer, a massive serpent-like demon appears, taps Glen's head, touches his hand, and retreats, leaving an eye embedded in his palm.
Driven by urgency, Glen removes the eye before descending the stairs, only for the demon to emerge once more. Utilizing a battery-powered launcher, he manages to fire the rocket into the demon, resulting in an explosive release that dissipates the stormy clouds above the house.
Triumphant, Glen returns inside to discover Angus has miraculously returned to life, followed closely by Al and Terry, who are unharmed. The trio now faces the daunting task of explaining the chaotic scene to their unsuspecting parents.
