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.

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.

As the islanders prepare for a fateful birth, Helen reveals her true intentions in coming to Osea and, after joining a search party, is pressed on her past. A peculiar girl befriends Ellie and offers to take her to a mysterious place closed off to outsiders.

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. As tensions reach a fever pitch for all those on Osea Island, Helen and her girls make a desperate final attempt to escape.