Bea, portrayed by Rose Leslie, and Paul, played by Harry Treadaway, are freshly wed and eager to enjoy a romantic honeymoon in a charming, secluded cabin nestled within a tranquil forest. Their getaway takes an unsettling turn when they visit a small restaurant run by a man who initially seems unfriendly and insists they leave. However, tensions ease as the owner realizes he is actually Bea's childhood friend, Will. Just as peace is restored, Will's wife enters, acting peculiarly and urging them to leave.
Everything seems to return to normal until their idyllic escape is shattered by Bea’s sudden disappearance. Paul discovers her wandering in the woods, naked and disoriented, with no clues as to her strange state. When they return to the cabin, she dismisses the incident as sleepwalking, attributing it to stress.
As days go by, Bea's behavior grows increasingly bizarre and alienating. Initially, Paul suspects her unsettling demeanor stems from her past encounter with Will, but it soon becomes clear that something far more sinister is at play. One night, while Paul lays awake, he notices mysterious bright lights outside but finds nothing upon investigation. As he grapples with Bea's escalating oddities, he notices peculiar markings on her inner thighs, which she brushes off as mosquito bites.
In a particularly alarming moment during a fight, Bea barricades herself in the bathroom. When Paul breaks down the door, he is horrified to find her in a frenzy, stabbing herself repeatedly in the genitals. In a desperate act, Paul ties her to the bed, only for her to plead for him to “take it out of her.” In an unsettling scene, Paul then reaches into her vagina and extracts a large, worm-like creature. Bea later recounts that during her time missing in the woods, she encountered the same enigmatic lights Paul had witnessed, which lured her into an encounter with shadowy figures that she believes impregnated her with this creature.
She confides in Paul that although she feels like herself, what's left of her is being taken away, and she feels compelled to protect him from being "disposed of." In a horrifying twist, Bea knocks Paul unconscious and transports him to the center of a lake. There, she secures an anchor to his feet and plunges him into the depths, believing this act of hiding him will safeguard him.
As the film progresses, Bea’s physical state declines dramatically; her skin becomes pale and flaky. Ultimately, she encounters Will's wife again, and together they walk towards the same mysterious lights. The film culminates with Bea declaring, > "Before I was alone, but now I'm not."
