Adam, a British conservationist specializing in plant and fungal life, his wife, Clare, 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, Clare, removing metal bars from the windows, watches as a man from the village, Colm, drives up looking for Adam. Clare 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. Clare 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 Clare know about 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. Clare at the house is frightened by Colm when he comes into her home uninvited, Colm warns Clare 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 trunk, he is hit and pushed in. After awaking in the trunk, 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 Clare is frantic for Adam and Finn and is relieved when they arrive. Adam immediately gets a gun and asks Clare to go upstairs and call the police. The lights are cut, and Adam goes downstairs to check the house, only to find the house is ransacked. Adam, believing it's Colm, becomes angry, and Clare begs him to leave. They pack up and make a run for the car. At the car, Adam and Clare 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. Clare 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 Clare before Adam is able to get the generator started. Now with the house in full light, Clare and Adam go about barricading the house with the same bars Clare 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. Clare, 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. Clare 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 Clare 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 Clare begin to fight, and Clare stabs Adam before running away with Finn.
Adam searches for Clare and Finn but starts to turn into a creature himself, the infection spreading from his eye. Clare defends herself and is able to get away to the forest. Adam tracks Clare into the woods, but hears a baby cry and follows the sound. In a nearby cave, Adam faces the creatures from the hallow. Clare 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 Clare 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. Clare, in the woods, sees the creatures coming towards her and uses a 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 Clare to believe him. She grabs the real Finn and runs away while Adam is 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. Clare, able to make it back to the house, breaks down and cries with the real Finn.
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.Joseph Mawle as Adam Hitchens
Bojana Novakovic as Claire Hitchens
Michael McElhatton as Colm Donnelly
Michael Smiley as Garda Davey
The film was written to segue from a relationship drama into more of a dark fairytale, and the pacing matched this transition. Hardy wanted to touch upon many different subgenres of horror, including body horror and creature feature. Inspirations for the film were Hardy's love for fairytales, Ray Harryhausen (with whom Hardy was close friends), and horror films like The Evil Dead, Alien, and The Thing. It was pitched as "Straw Dogs meets Pan's Labyrinth". Although Hardy is a fan of vampires, werewolves, and zombies, he felt that there were already enough films based around those monsters, and he wanted to do something less overexposed. He decided on using Irish folktales as a base. As a fan of Mawle, Hardy wrote the lead role for him.
Shooting took place in Ireland for six weeks. Hardy wanted to keep the shooting "as real as possible". This involved shooting on location in forests and around lakes, including one scene where Hardy wore a wetsuit to shoot from inside a lake. Continuing this theme of realism, Hardy focused on giving the narrative a more rational, scientific base than the traditional magic-based fable. Martijn van Broekhuizen was the cinematographer. van Broekhuizen and Hardy had not worked together previously, but van Broekhuizen was aware of his work. After talking together on Skype, van Broekhuizen was impressed with Hardy's ability to clearly express his vision. van Broekhuizen credited his work on a Dutch film in which he shot complex nighttime scenes in a forest as the reason why he was hired for The Hallow. Shooting took place with a Arri Alexa digital camera, and the lighting was designed to evoke a fairytale atmosphere. Hardy had wanted to shoot on film, but budget limitations prevented this. Hardy characterized the production as having both "good fear" and "bad fear": the good kind inspired them to perform to the best of their ability, and the bad kind caused them to be averse to taking risks. Hardy credited his confidence in the production as raising morale when they were performing long shoots in rural locations.
The creatures were done by British SFX artist John Nolan, whom Hardy sought out in effort to find what he called the British equivalent to Stan Winston or Rob Bottin. The effects were primarily practical. The creatures were a mix of practical effects and CGI. Although a fan of old-school "man in a rubber suit" effects, Hardy took advantage of modern technology to augment the look of the creatures. In order to make the creatures more unsettling, their limbs were extended using practical effects, as Hardy believed a full-CGI monster would not be scary. This allowed the filmmakers to use on-set lighting to heighten the creepiness. This was inspired in part by Alien. The baby seen in the film is a mix of animatronics and twins that were digitally shot in front of a blue screen. The effects were inspired in part by the 2001 version of Planet of the Apes, which Hardy said "mixed those gritty real environments with the slick performance capture to blur the lines".
Rotten Tomatoes, a review aggregator, reports that 71% of 51 surveyed critics gave the film a positive review; the average rating is 6.2/10. The site's consensus reads: "Drenched in dark atmosphere and bolstered with some real emotion underneath all the chills, The Hallow suggests a wonderfully horrifying future for director Corin Hardy." It has a score of 65/100 on Metacritic based on 12 reviews. Geoff Berkshire of Variety wrote, "It takes time for The Hallow to get rolling, but once it reaches a bang-up final act, genre fans could walk out clamoring for a sequel." David Rooney of The Hollywood Reporter wrote, "As first films go, this one is visually energized, dynamically paced and discerning in its cine-literate references. It's also a kickass calling card for Hardy as he moves on to bigger projects." Michael Roffman of Consequence of Sound rated it C− and wrote, "Despite a number of supremely original sequences – one of which involves a car trunk, another a cavernous dwelling – they’re all carried out rather predictably, cinching any warranted tension within seconds." Fred Topel of Bloody Disgusting rated it 4/5 stars and called for sequels to revisit the film's mythology, which he praised.