Mina, an American residing in Ireland, finds herself working at a pet shop, grappling with the emotional aftermath of her mother's tragic death, an incident for which she feels responsible since childhood. As time passes, her relationship with her twin sister becomes strained. One day, her boss assigns her the duty of delivering a precious bird, whom she affectionately names Darwin. While driving through a dense forest, however, fate intervenes, and her car breaks down, leaving her stranded.
As Mina ventures into the woods, she discovers her car has vanished upon her return. During her search, she encounters an enigmatic old woman named Madeline, who seems to be watching her intently. Hearing unsettling, inhuman sounds echoing through the forest, Mina decides to follow Madeline, who leads her to a bunker-like structure known as 'The Coop.' Inside, Mina meets two other inhabitants, Ciara and Daniel, who have also been ensnared in this eerie situation. Madeline reveals the ominous nature of their existence — a mysterious group known as 'The Watchers' surveils them from behind mirrored glass every night.
Determined to escape the confines of the forest, Mina tries to leave the following day, only to discover there is no way out. Madeline lays down the harsh rules of their confinement: The Watchers will kill anyone who ventures outside the Coop after dark; they have a dread of light, and the Burrows — a series of tunnels within the forest — are strictly off-limits, as that’s where the Watchers lurk during the day. Mina forms a bond with her newfound companions, learning about their months-long entrapment and the absence of Ciara's husband, John, who had left the Coop in search of help.
Tension arises between Mina and Madeline, whom she sees as overly compliant with the Watchers. In a moment of rebellion, Mina and Daniel explore a Burrow and unearth various items, including an old bicycle and a camcorder. Their situation escalates upon witnessing a Watcher burn itself in the sunlight, prompting the group to decide to capture evidence of the Watchers using the camera. An unexpected arrival of John at the Coop is met with suspicion from Madeline, who believes he is a Watcher. When "John" fails to prove his identity, chaos ensues, leading to an unsettling encounter.
As the days turn into months and winter sets in, the toll of their ordeal becomes increasingly apparent. Relationships fray, culminating in Daniel locking Mina and Madeline out of the Coop during the night. They witness the wrath of the Watchers, humanoid creatures that mimic the Coop's occupants, and barely manage to return to safety. In a shocking revelation, Madeline explains that these entities, referred to as 'Changelings' or 'Faeries,' are desperate to learn how to emulate human behavior. Frightened by Mina’s blatant disregard for the established rules, the Watchers threaten the Coop, and the group uncovers a hidden passage beneath it, leading to another bunker filled with supplies and distressing video diaries of a professor who initially aimed to study the Watchers.
The professor's last entries detail his disillusionment, revealing the darker side of his research and his tragic fate, ultimately giving Mina and her companions guidance for their escape. The following day, they emerge to find the Coop ransacked, forcing them to put the professor's advice into action. They escape with Darwin's assistance, but tragedy strikes when Daniel encounters a Watcher imitating John and meets a grim end.
Mina, Madeline, and Ciara finally find their way back to civilization, but the haunting memories linger. In the aftermath, when Mina visits the professor's university to eliminate any traces of his work, she uncovers the history of The Watchers, once peaceful with humans before the conflict drove them underground. She learns about hybrids, beings part human and part watcher, who roam the world during daylight.
Mina later reconnects with Ciara, showing her photos that unveil Madeline's tragic backstory — once the professor’s wife, she perished before entering the forest. However, the true shock comes when the real Ciara arrives, and Madeline reveals her presence by cycling through the forms of those they lost, before attempting to attack Mina. In a dramatic confrontation, Mina manages to reach Madeline, urging her to let go of her grief. This interaction triggers a transformation, causing Madeline to embrace her true identity as a Watcher.
As time passes, Mina finds closure with her sister, overcoming her own mourning. Meanwhile, Madeline, now taking on the guise of a child, quietly watches over Mina, a bittersweet reminder of their intertwined fates.
