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. As he meets Osea's secretive locals, Sam grows increasingly desperate to return to the mainland to make an important call - but the island proves difficult to escape.

Sam is shocked to learn the truth about his connection to Osea and the islanders’ intentions for his future. Later, in a fight for his life, Sam tries desperately to escape their clutches and make it back to the mainland in one piece.

Winter. 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.