A family who moved into a remote mill house in Ireland finds themselves in a fight for survival with demonic creatures living in the woods.
A couple and their infant move into the backwoods. The husband, an expert in microbiology, has come to inspect the trees for clearance. He is warned by the locals not to trespass the forests as the "hallows" will trespass into his house and steal his baby. One night the window of the infant's room is broken, but the local cop assures the couple not to worry, some bird must have done it. But things get dreadful as the mystery unfolds.—[email protected]
Adam, a British conservationist specializing in plant and fungal life, his wife, Claire, and baby son Finn, travel to a remote Irish village surrounded by a large forest.
Some time later, Adam walks through the forest with baby Finn, surveying the woodland for plans to log the area, and stumbles across an abandoned house with the carcass of an animal that has been burst open from a fungus-like substance. He takes samples of the carcass before heading home. Back at the cabin, Claire, removing metal bars from the windows, watches as a man from the village, Colm, drives up looking for Adam. Claire informs him that Adam is not presently there, and the man leaves angry.
That night, the baby begins to cry, and they hear a crash. Claire runs to the room, but the door mysteriously slams shut. Once the door is opened, they realize the window was smashed and call the police, believing it may be Colm from the village. The police arrive, but believe a bird flew into the window causing the damage. The officer tells both Adam and Claire about the legend of the surrounding forest; the villagers claim it belongs to "The Hallow" and is inhabited by "fairies, banshees and baby stealers." The incident is dropped and the police leave. Adam goes outside to take pictures of the damage and notices odd movement in the woods.
The next day, Adam, with Finn, travels to town with the damaged window to have it repaired and is again warned about the hallow. Claire at the house is frightened by Colm when he comes into her home uninvited, Colm warns Claire to leave the village and leaves behind an old book. On his way home from the village Adam has issues with his car, forcing him off the road and nearly causing him to wreck. He opens the hood to see a vine-like fungus clogging the engine, opening the boot, he is hit and pushed in. After awaking, Adam hears noises outside scratching at the car, with Finn, still in the car, beginning to cry as the car shakes and shadows move menacingly across the windows. Adam kicks a hole in the backseat cushion and pushes his way out just as the car stops shaking. He gets Finn out of the car, which has a number of deep scratches across it. Then, scared and worried, he starts towards home.
At home Claire is frantic for Adam and Finn and is relieved when they arrive. Adam immediately gets a gun and asks Claire to go upstairs and call the police. The lights are cut, and Adam goes downstairs to check the house, only to find it ransacked. Adam, believing it's Colm, becomes angry, and Claire begs him to leave. They pack up and make a run for the car. At the car, Adam and Claire attempt to get it started and clear the fungus - which has grown throughout the engine at an amazing pace - from the vehicle, as the creatures from the forest start to chase after them. They manage to start the car, but the creatures pelt it with a mud-like goo - ostensibly, the fungus - which breaks the glass portions of the car, causing them to crash into a ravine. Unable to go any further, they make a run back to their house.
Back in the house, Adam is stabbed in the eye with a syringe-like appendage while looking through a keyhole outside. It is then discovered that light can keep the creatures at bay. Claire heads to the attic and Adam attempts to get the back-up generator started. The creatures get into the house and go after Finn in the attic, almost reaching Claire before Adam is able to get the generator started. Now with the house in full light, Claire and Adam go about barricading the house with the same bars Claire removed from the windows and pointing lamps at them. To protect Finn, Adam places him in a locked cupboard and points another light directly at it. Adam looks through the book Colm left and reads information on the forest legends. Claire, worried about Finn, opens the cupboard to find a creature kidnapping Finn, and they chase after the creature, but it is able to get away when Adam falls from the attic and is knocked out and left with a broken leg. Claire chases after Finn and finds him in a pond, but is able save him before he drowns. Going back to the house, Adam, now awake, sets his broken leg and helps Claire with Finn. Adam becomes suspicious of Finn, believing he is not actually Finn but a changeling, and that the creatures in the hallow stole Finn and exchanged him with one of their own. Adam and Claire begin to fight, and Claire stabs Adam before running away with Finn.
Adam searches for Claire and Finn but starts to turn into a creature himself, the infection spreading from his eye. Claire defends herself and is able to get away to the forest. Adam tracks Claire into the woods, but hears a baby cry and follows the sound. In a nearby cave, Adam faces the creatures from the hallow. Claire makes it out of the forest to the next house, where Colm resides, and begs to be let in, but Colm, answering the door with a shotgun, tells her of his daughter Cora, who was also taken by the creatures. He tells her to get away from his home before they come, and Claire is forced to run away with Finn. Adam finds the baby in the cave being held by a transformed Cora. Adam understands what the creatures want Finn for, they want him for their family. Claire, in the woods, sees the creatures coming towards her and uses camera flashes to keep them away. As she moves away, Adam is able to find her and brings her the second Finn. Adam keeps the creatures away long enough for Claire to believe him. She grabs the real Finn and runs away while Adam is fatally wounded by another creature. As daylight breaks, Adam sees the changeling Finn lose consciousness peacefully, as if sleeping, before being gruesomely destroyed by the light of the rising sun. Claire, able to make it back to the house, breaks down and cries with the real Finn over her husband's death.
As the credits roll, a logging company is cutting down the forest. The screen pans over to the forest as a truck containing several felled trees rolls up and stops. The fungus is seen on the trees and visibly spreads to several more logs before the screen cuts to black. Suddenly, the hallow appears screaming on the screen before it fades to black.
