A couple travels to Sweden to visit a rural hometown's fabled mid-summer festival. What begins as an idyllic retreat quickly devolves into an increasingly violent and bizarre competition at the hands of a pagan cult.
When her mentally ill mother passes away, a woman named Annie (Toni Collette) and her husband (Gabriel Byrne), son (Alex Wolff) and daughter (Milly Shapiro) all mourn her loss. The family turn to different means to handle their grief, including Annie and her daughter both flirting with the supernatural. They each begin to have disturbing, otherworldly experiences linked to the sinister secrets and emotional trauma that have been passed through the generations of their family.