Set on an isolated island off the Scottish coast, in a community run by women who are in awe of a mysterious structure called the Thrashing House, the novel is narrated by two teenage girls in very different circumstances. Mary is doing her best to protect her younger brother, Barney, as the island's sons are mysteriously disappearing. Morgan is scheming to escape the prison her parents have made of their home. The two girls unite, each on a desperate mission in which secrets will be revealed and lives changed forever.