In the mid-1700s, a well-to-do British family and their captain shipwrecks on an island in the Atlantic on their voyage to America. As they gather food and supplies to build a raft, the son is kidnapped by a mysterious figure and brought to a hidden commune inhabited by a gang of cutthroat pirates led by their king Blackwater Jack. As the cunning monarch grooms the boy as the successor of his colony, the family, armed with limited weapons, pushes on through thick and thin to get him back, waging a dangerous war with the buccaneers.