Plot

"You've seen horrible things, an army of nightmare creatures. But they are nothing compared to what came before, what lies below." This article may contain major plot spoilers. Proceed with caution.

Prologue

Gary Sitterson and Steve Hadley are in a breakroom getting coffee talking about Hadley's domestic issues. Wendy Lin arrives and tells them Stockholm failed and now it's just them and Japan who are left to do the job. Sitterson and Hadley remain complacent, stating that Japan is number one and has never failed, while they are number two since they always try hard. They leave for the control room.

Passing Harbinger

As Dana Polk packs for a trip, she finds a sketch of a professor she had been seeing. It is implied from her conversation with her friend Jules Louden that Dana had recently dated and slept with her college professor, who later broke up with her via email. Jules' boyfriend Curt Vaughan introduces Dana to Holden McCrea, one of Curt's teammates on the football team. They wait for Marty Mikalski to arrive and they in their RV to Curt's cousin's cabin in the woods.

Before they reach the cabin, they pull over at an old gas station, where they meet Mordecai. Mordecai tells them that they'll have enough gas to get to the cabin, "but getting back, that's your concern.". When Mordecai mentions that he has been living there since the end of the war and rudely scolds Jules when she asks which one, Marty snidely implies that it was the civil war. Mordecai insults Jules by calling her a whore and Curt throws money at him and they leave, but not before Marty pulls an insult on Mordecai.

Entering the Cabin

The creepy painting in the bedroom that conceals the one-way mirror.

The group drives through an enclosed mountain tunnel that serves as the only entrance and exit to the woods surrounding the cabin.  Once they enter the tunnel, a hawk is seen, unbeknownst to the group, gliding across the chasm that separates the two tunnel openings.  As the camera follows the flying hawk, it collides into what appears to be an invisible, electrified wall.  The hawk tumbles down the seemingly bottomless chasm and out of sight before the group emerges from the tunnel, and they continue.  

The group finds the cabin, and begin to unpack in separate rooms. In Holden's room, he removes a disturbing painting to finds a one-way mirror into Dana's room. After he tells Dana, they decide to switch rooms, and the group prepares to begin their summer vacation.

Choosing the Monster

Mordecai calls the Organization to give a deep monologue about being serious with the job, only to be laughed at by Sitterson and Hadley. Lin tells them that the Chemistry Department is prepared and had even managed to plant chemicals in Jules' hair dye to make her sexually active. Organization employees gather in The Facility Control Room for a wagering pool on which monster(s) will be selected. Sitterson takes bets and teases Hadley for betting on the Merman, which Hadley really wants to see, but is an apparently unlikely scenario.

In the cabin, night has fallen and personality changes began to develop in the group as they play Truth or Dare. A cellar door pops open mysteriously (remote-controlled by the underground technicians), revealing a cellar where the group find various strange artifacts. Ignoring Marty's suggestion to go back upstairs, they investigate the different items. Curt nearly blows a conch shell, but puts it down. An intense scene plays wherein everyone begins fiddling with different objects: Curt plays with a puzzle ball, Holden operates a jewelry box with a dancing ballerina, Jules removes a necklace from a bridal gown and plans to wear it, and Marty begins looking through an old film strip. They all stop, however, when Dana calls them over to read from a diary of one of the members of The Buckner Family, the cabin's supposed original owners. They come across a Latin incantation at the bottom. Nobody but Marty notices a disembodied whisper urging them to "read it out loud." Despite Marty's objections, Dana reads the Latin aloud and thus unwittingly summons the zombified Buckners from a nearby grave.

The Death of Jules

Affected by mood-altering drugs applied by the underground technicians, Curt and Jules go off into the woods and begin to undress. The underground technicians focus intently on the exposure of Jules' breasts, raucously at first but later soberly observing that her disrobing for the audience is an essential part of the ritual. "Gotta keep the customer satisfied."

Curt and Jules hesitate to have sex, however the technician released a pheromone fog and successfully induced them to have sex. As Curt and Jules are reaching orgasm, the Buckner Family attacks. They stab Jules' hand and wound Curt in the shoulder. They hold Curt and make him watch as Mother Buckner cuts Jules' throat (and eventually beheads her) with a rusty saw.

Meanwhile, in The Facility, Hadley and Sitterson mark her passing with a prayer: "This we offer in humility and fear, for the blessed peace of your eternal slumber, as it ever was."

Splitting Up

The underground technicians pull a lever operating a complex machine that pours sacrificial blood into the grooves of a stone carving.

Marty is in his room smoking pot and reading a picture book. He hears a mysterious whisper suggesting he go for a walk. He is disoriented and cries out, "You think I'm a puppet, huh?... I'm the boss of my own brain so give it up!" However he relents and goes for a walk.

Just outside the cabin, Marty unzips and begins to relieve himself while Patience Buckner stalks him in the background. Suddenly, Curt grabs Marty and urges him back into the house, where Curt tells the group that Jules is dead. Matthew Buckner appears in the doorway and tosses Jules' severed head into the room. They push him out and bar the door. Curt outlines a desperate plan for everyone to work together, sealing the house against intruders -- the kind of practical things that a horror-movie audience would urge the onscreen victims to do.

Underground, Sitterson releases gas into the cabin that disorients the group. Curt and Holden now agree that it would be best for everyone to split up. "We can cover more ground that way." Only Marty is incredulous. The Buckners begin to attack the house and everyone runs to his/her own room, where the technicians lock them in.

Discovering the Fourth Wall

Reaching to close the window, Marty knocks over a lamp and breaks it. He discovers a spy camera was hidden in the lamp, and locates the signal feed embedded in the wall. He pauses for a moment and exclaims, "I'm on a reality TV show!" Realizing they are on the verge of being discovered, the technicians prepare to gas Marty. But Judah Buckner attacks and pulls Marty out through the window. Marty puts up a spirited fight, but Buckner stabs him with a trowel and drags him away screaming. Offscreen, blood splatters with the sickening squish of the trowel striking flesh.

As Sitterson and Hadley pull the next lever to pour blood onto another stone carving, a brief but heavy earthquake shakes the area, confusing the kids and surprising the facility workers below.

Escaping the Cabin

Mother Buckner attacks Dana in her room, reaching through the window. Dana is locked in and calls for help. Holden breaks the one-way mirror between their rooms and helps Dana get through. When the Buckner Zombies begin to attack Holden's room, Holden and Dana lift a bed to block the window and discover beneath the bed a trap door. They call to Curt, who is in his room next door, to find a way into the basement so they may escape.

Holden and Dana climb through the trap door and into a room that Dana recognizes as the Black Room referenced in Patience Buckner's diary. Knives, axes, traps and other weapons lie everywhere. Matthew Buckner attacks Holden from the trap door opening above, but Dana grabs a crowbar, jams it through his eye, and then stabs him with a knife until he is incapacitated. Sitterson flips a control switch that applies a small electric shock to the knife handle so that Dana drops it without noticing. Curt appears from an adjoining basement room and leads the group outside to the Rambler. They drive off in the Rambler, failing to notice a bloody handprint on the door.

Crash and Burn

The "floaty girl" being defeated by the japanese school girls performing a ritual.

Sitterson watches the video feed from Kyoto as the schoolgirls unite against the Japanese Floaty Girl and sing an incantation that defeats the ghost and reduces her spirit to a frog in a bowl. Hadley, Sitterson and Wendy Lin commiserate on Japan's failure ("Zero fatalities, total wash") which leaves North America as the only viable scenario for a successful ritual.

Glancing at the monitor, Hadley realizes that the RV is about to escape through the mountain tunnel and that the expected cave-in has not occurred. They urgently contact Demolition and learn of a "glitch" and communications failure. Sitterson sprints to the Demolition control room and patches the wires at the last second to detonate explosives for the cave-in. Curt is forced to back the Rambler out of the tunnel the way they entered.

With the tunnel blocked, a narrow but very deep ravine lies between the victims and their escape. Curt attempts to jump across the ravine on his motorcycle, intending to get help and return. But he crashes into the invisible force field and falls to his death. Hadley pulls the next lever -- more blood falls on another stone carving.

Holden and Dana drive the RV towards the cabin, hoping to find another exit road, but Father Buckner (who had been hiding in the Rambler) fatally stabs Holden while he's driving. The RV veers off the road and crashes into a lake.

"The Virgin's Death is Optional"

As Dana fights to escape the submerged RV and swims to the surface, the underground technicians celebrate. Truman asks how the ritual can be complete when the footage shows Dana is still alive. Hadley explains that the Virgin's death is optional, so long as she outlives the others and suffers in the ordeal.

Dana climbs onto the pier and is attacked by Matthew Buckner. Their struggle is displayed on monitors all over the underground control room where Organization workers have gathered for cocktails. While Buckner beats Dana and tosses her about onscreen, various employees discuss overtime pay, hit on each other, and relish their success. Hadley is pleased with the outcome but still disappointed that he didn't get to see the Merman. Sitterson happily greets the Demolition team, but they remain defensive about the near-miss with the cave-in. They reiterate to Sitterson that it wasn't their fault, that the delay was caused by a "power rerout from upstairs".

The Fool Survives

Sitterson reacts with dread to the Demolition team's comment about interference from "upstairs". At that moment, the direct hotline phone rings and Hadley answers in a hushed voice. He learns that someone thought dead has actually survived and thus the ritual isn't complete. He turns to look at the monitor and asks sternly: "Which one?"

On the pier, Matthew Buckner prepares his final blow to kill Dana, but Marty appears and stops him. Marty leads Dana to the Buckners' grave, where Judah Buckner lies in pieces, but still alive and moving. Marty confesses "I had to dismember that guy with a trowel."

Marty shows Dana that the grave is connected to an underground elevator, and surmises that someone sent the Buckner family to kill them. Various pieces of Judah Buckner fall into the elevator as they climb in. Marty, Dana, and the Judah pieces ride the elevator down into The Facility.

Going Down

Dana and Marty discover that the elevator car is a transparent cube that moves on horizontal tracks as well as vertical. They are startled by monsters in adjacent cubes, first a Werewolf and then a Wraith, and then the Hell Lord and the Sugarplum Fairy. Dana notices that the Hell Lord is holding a puzzle ball like the one Curt was playing with in the basement. She realizes that the artifacts in the basement were a way of forcing them to choose the manner of their own deaths. Dana beats angrily at the walls of the cube as hundreds of other monsters and nightmare creatures, each in their own cubes, surround her in the Facility's elaborate prison.

Wendy Lin explains that her Chem team treated Marty's marijuana to make him more susceptible to their mind-control drugs, but the team must have missed a stash, and smoking it has immunized him against their influence. Hadley and Sitterson locate Marty and Dana's cube and send it down to the elevator lobby, giving orders to kill them both -- Marty first. A guard confronts them with his gun drawn, but is distracted by Judah Buckner's still-moving severed arm. Marty and Dana subdue the guard and escape with his gun and Judah's trowel.

System Purge

Marty and Dana step into the Elevator Lobby, where the Director speaks to them over the intercom, urging them to give up. She tells them that their deaths are inevitable and necessary, part of something bigger and older than they could ever imagine.

The Security Guards about to be attacked by the monsters during the Purge.

As an armed security team approaches the lobby, Marty and Dana duck into the elevator control booth. The security team opens fire on them, and the two take shelter behind the control booths bulletproof glass.  Dana discoveres the System Purge button, and after a moment of hesitation, says "An army of nightmares, huh?  Let's get this party started."  She presses the button, releasing all the caged Monsters into the Facility. The unleashed monsters slaughter the security team and soon make their way across the facility, causing chaos and killing everyone.

Dana and Marty hide in the elevator control booth until flushed out by a Dragonbat. They escape through the melee and into a large hole made by the bat that opens into a secret escape tunnel.

The Control Room is the Organization's final hold-out, but Truman is killed by the scarecrow folk when he detonates his grenade and Hadley finally gets to see the Merman -- right before it kills him. Sitterson opens the computer padlock to the escape tunnel, and climbs down just as Wendy Lin is snatched from above by giant tentacles. Sitterson runs into Dana, who accidentally stabs him. Dying, Sitterson pleads with Dana to kill Marty.

Meeting the Director

Marty and Dana reach the Ritual Chamber, where they observe five stone carvings depicting the horror movie archetypes: The whore, the athlete, the scholar, the fool and finally the Virgin. Dana realizes that she and her friends were the victims in a ritual sacrifice, not merely to be killed, but also to be punished.

The Director arrives and tells them that the sacrifice is to appease the Ancient Ones -- giant, evil gods who slumber beneath the chamber and will awaken to destroy the Earth if the ritual is not completed. Marty protests that the world isn't worth saving if it must subsist on such barbarism. But Dana understands she can survive only by killing Marty. She aims the gun at him, but a werewolf appears and attacks her, causing her to drop the gun.

Marty retrieves the gun and shoots the werewolf, causing it to retreat, but then he and the Director fight. Patience Buckner, who followed Marty and Dana from the surface down into the facility, arrives and kills the Director with a hatchet. Marty kicks them both off the ledge of the ritual chamber and into the Ancient Ones' nest below.

The End of the World

Marty and Dana share a joint and reflect on all they've been through. Dana apologizes to Marty for almost shooting him, and Marty apologizes for letting the werewolf get her and then ending the world.

As the chamber and entire facility begin to rumble and collapse, they speculate how great it would have been to see the "giant, evil gods" that are about to awaken.

A huge arm reaches up through the Facility and bursts out of the cabin at the surface as the Ancient Ones begin to arise to destroy the earth.