Adam, a dedicated British conservationist with a deep affinity for plant and fungal life, embarks on a journey with his wife, Claire, and their infant son, Finn, to a secluded Irish village enveloped by an expansive forest. Their tranquil getaway soon spirals into a nightmare when Adam, while surveying the woodlands for potential logging, stumbles upon an eerie abandoned house. Within it lies the gruesome remains of an animal, grotesquely ruptured by what appears to be a fungus-like entity. He collects samples, unaware that this discovery marks the beginning of their harrowing ordeal.
Back at their cabin, Claire is busy removing metal bars from the windows when a local man named Colm arrives, seeking Adam. Coldly informed of Adam’s absence, Colm leaves, furious at Claire’s refusal to cooperate. That night, a mysterious crash disrupts their peace, and in a panic, Claire rushes to investigate, only to find the door inexplicably slamming shut behind her. When they finally manage to open it, chaos ensues—they discover a smashed window, prompting a call to the police. They suspect Colm, but the officers dismiss their fears, attributing the damage to a bird. They further regale Adam and Claire with local legends about "The Hallow," a supposedly enchanted grove inhabited by sinister entities.
Undeterred, Adam takes Finn to the town the following day for window repairs, only to be duly warned about the dangers of the forest. Meanwhile, Claire faces her own ordeal when Colm barges into their home uninvited, advising her to flee. He departs, leaving behind an ominous old book. On his way back home, Adam encounters car troubles due to a vine-like fungus clogging the engine. As he struggles, the situation becomes dire when he is ambushed and trapped inside the car, while Finn begins to wail from within.
Upon managing to escape and return home, Adam finds the cabin ransacked, leading him to believe it was Colm’s doing. As the lights flicker out, he and Claire scramble to arm themselves. In a frantic race to their car, they witness the forest creatures springing into pursuit, forcing them to flee once more. Their vehicle careens into a ravine, and they make a desperate dash back to their house, but their troubles escalate when Adam suffers a debilitating injury whileTrying to secure safety.
Amongst the chaos, Adam discovers that light is the only means to repel the creatures. As Claire retreats to the attic with Finn, Adam works diligently to power the backup generator. Their makeshift sanctuary is soon besieged by the dark forces lurking outside. In a heart-stopping moment, while trying to safeguard Finn, Adam becomes convinced that the baby they have is a changeling—a deceptive creature swapped in place of their true son. This suspicion ignites a fierce confrontation between the couple, resulting in Claire wounding Adam in her desperate bid to escape.
As Adam grapples with his changing condition, he races to reunite with Claire and Finn. The relentless link between mother and child is palpable, even as she eludes the dangers lurking in the forest. In a climactic finale, Adam finds himself in a cave, facing the very creatures that want Finn for their own. Claire, in a moment of desperation, employs camera flashes to push back the entities, believing in Adam's revelation just in time. Tragedy unfolds as Adam sacrifices himself to keep the monstrous beings at bay, ensuring Claire's safe return with their real son.
In the aftermath of their grim battle, the film concludes with Claire mourning Adam's sacrifice as the reality of their loss sets in. The final scenes shift to a logging company unknowingly contributing to the forest’s haunting legacy, as the pervasive fungus begins to reclaim its territory, leaving viewers with a chilling image of the forest and its dark inhabitants, signifying that the threat of "The Hallow" lingers on.
