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.

Plagued by a disturbing dream, Sam awakes to find himself in a compromising situation and, having missed the morning crossing to return to the mainland, 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 as the islanders prepare for a dry run of their big event.

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.