Adam Hitchens, a London-based conservationist, takes a job surveying an Irish forest, and moves his wife Clare and baby son Finn to a millhouse deep in the idyllic countryside. The locals warn Adam of the consequences if he disturbs the ancient, sacred forests rooted in Irish folklore. When someone or something breaks into the house at night with the apparent intention of harming their child, the worried couple believes it to be the locals, intent on driving them away. But after the intruders seem to elude the police, the millhouse comes under attack by horrific, vicious beings that emerge from the sacred woods. Stranded and alone, Adam and Clare must fight to protect their child and survive the invasion.