After Sam, a London family man in crisis, rescues a troubled young girl in the woods, he brings her home to the ominous Osea Island where inhabitants are preparing for a curious annual festival involving the island's history.

Having missed the crossing to return to the mainland, Sam spends the day trying to learn more about the islanders' beliefs. After a series of confusing and terrifying events leaves him shaken, Sam quickly plots a way off Osea with Jess but is convinced to stay put.

Helen surprises her two daughters, Tallulah and Ellie, with a trip to Osea Island for Ellie's birthday. After finding that the cottage she booked is suspiciously no longer available, Helen scours the island for a place to stay but finds resistance at every turn.

With a battle for power brewing, Helen comes face to face with what she's been searching for since arriving on the island. When the sisters are separated, Tallulah witnesses a gruesome act of violence, and Ellie learns of their mother's hidden agenda.