The film begins with two middle-aged men casually strolling through a facility while discussing trivial matters related to marriage and impending parenthood. Meanwhile, a younger woman clad in a lab coat eager to inform them about a mysterious incident at the Stockholm facility finds her words largely ignored. They express confidence in the Japanese branch's capabilities, playfully noting that their American facility typically ranks second. After hopping into a golf cart, the younger man invites his older companion over for a weekend visit.
The narrative then shifts to a college-town setting, where Dana, played by Kristen Connolly, and Jules, portrayed by Anna Hutchison, engage in discussion about their academic lives. Dana reveals a troubled past involving an affair with a professor that ended poorly. Curt, played by Chris Hemsworth, Jules' boyfriend, arrives with a casual throw of a football, which inadvertently lands out the window. Holden, played by Jesse Williams, catches the ball below and is subsequently introduced as a potential romantic interest for Dana. The group is preparing for a long weekend at a lakeside cabin owned by Curt's cousin. Just as they are about to leave, their friend Marty, who is emblematic of a carefree spirit, turns up in a car, heavily indulging in marijuana from a unique bong that doubles as a travel mug.
As the group travels closer to the cabin, we cut back to the aforementioned facility, where the two middle-aged men, Sitterson (Richard Jenkins) and Hadley (Bradley Whitford), observe the kids from their surveillance screens. They seem to be administering various substances to the young adults through unexpected means—such as hair dye for Jules and Marty’s stash—without providing any insight into their ulterior motives.
The group stops for gas and directions where they encounter Mordecai (Tim DeZarn), an eccentric attendant whose unhinged remarks about safety give the teenagers pause, yet they choose to wave off his odd warnings with laughter. On their way to the cabin, they traverse a U-shaped tunnel. As they do so, a bird tragically collides with an invisible barrier, leading to its shocking demise.
Back at the facility, Hadley answers a call from Mordecai about the group’s impending arrival, but an amusing miscommunication leads to a fallout when Mordecai explodes with frustration at being put on speakerphone. Upon reaching the cabin, the friends start to unpack, blissfully unaware of the cameras documenting their every move. Holden finds a hideous painting in his room that conceals a window into Dana's quarters, inadvertently exposing her as she undresses. Upon realizing the situation, Holden rushes to alert Dana, leading to a room exchange that stirs romantic tensions.
Meanwhile, the facility staff places bets on the unfortunate fates that await the group in the cabin, revealing a stark contrast between the laid-back antics of the young adults and the cold calculations of the observers. Amidst revelry, a game of Truth or Dare unfolds, leading Dana to unearth an ominous diary in the basement that belonged to a girl who suffered grievous abuse. Despite Marty's apprehensions, Dana reads a Latin incantation from the diary, unwittingly summoning a horrifying family of zombies from their graves.
As the killings begin—first for Jules, whose fate leads to some ritualistic mumblings from Hadley and Sitterson—the group finds themselves in a fight for survival. Marty stands apart, experiencing visions of the truth behind their circumstances while attempting to rally the others as their world begins to shatter around them.
As the jaunt through horror continues, layers of betrayal, supernatural mayhem, and the hidden machinations of a larger scheme come to light. Dana and Marty face off against harrowing obstacles while trying to escape; they must navigate a facility that is perhaps more dangerous than the monsters outside.
In a climactic finale, the depth of this twisted ritual is unveiled, as a supernatural force threatens to emerge, culminating in Dana making a fateful decision—even as the lines between good and evil blur. The film artfully intertwines threads of horror, existential dread, and dark humor, ultimately leading to a showdown that leaves the audience questioning the cost of survival.
With chaos erupting and the ground trembling, a giant hand breaks free from the earth, suggesting that the ancient and dark forces of which they spoke are now on the verge of breaking free as the protective barrier crumbles.
