After a warning not to come to their family lake house in Maine, Lyla and Ed bring their brother Martin back into the family conversation, intending to stage an intervention for a man who is convinced the military has experimented on him. They arrive to find Martin spiraling into paranoia, convinced that a monster is trapped in the basement. He explains that he found a mysterious pod while hunting in the woods and believes it to be part of a government plot to create artificial, inanimate killers. In a troubling move, Martin has pulled several of his teeth in a bid to uncover a supposed tracking device, and though he destroyed it, he cannot prove his theory to his skeptical siblings.
Ed, a mental health professional, argues that Martin should be taken back to the VA hospital where he was once institutionalized after an attack on a nurse. Lyla, however, wants to hear Martin’s side of the story more fully, hoping for clarity in a situation that feels dangerously off-kilter. As the siblings debate, Martin overhears them discussing a plan to institutionalize him again and confronts them with a knife. In the tense moment that follows, Martin declares that they are all going to die anyway and slashes his own throat. Ed fights to stop the bleeding, while Lyla, struck with fear, acts on instinct to call for help, only to discover that Martin has severed the phone cord and sabotaged the car as well.
Martin dies from his blood loss, and Ed kicks open the barricaded basement door to inspect what’s below. Lyla pleads for him not to descend, but Ed feels compelled to check for an injured person he believes might be held captive. Darkness greets him at first, then a flicker of light reveals a humanoid creature—the thing Martin claimed to have warned them about. The creature launches an attack, and Ed barely manages to escape back into the house. Realizing the danger, Lyla urges him to leave the house, while Ed, disoriented by memories of Martin’s talk of a hallucinatory venom, fights the monster with a combination of wit and force, using a syringe with a mild sedative he had intended for Martin to slow the creature, then retrieves Martin’s rifle and prepares for a final stand. The creature recovers and rushes him, and Ed shoots it.
Meanwhile, Lyla discovers that the car still won’t start and heads toward a neighboring house, banging on a door in a desperate bid for help. A passing motorist named Smith stops, questions her calmly, and, in a disturbing twist, dismisses the notion of any government conspiracy. He shoots Lyla and kills her, leaving her fate sealed off from Ed.
Back at the house, Ed, delirious with triumph, revels in what he believes is the end of the threat, repeatedly striking the creature with the rifle butt to ensure it is dead, and calls out for Lyla’s return. Smith, who has been assisting from outside, shoots Ed in turn and reports via walkie-talkie that the creature is dead—but he does not know whether another one might be close by. After sharing coordinates, Smith is distracted by a noise behind him, and the creature, though believed dead, attacks and kills him as he turns around.
In the end, the apparent monster remains a threat, and the eerie suspicion that more of its kind could exist elsewhere lingers, leaving the family trauma and the mystery of the pod unresolved in a chilling, unresolved close.
