Claire is only seven years old when her college-age sister, Alison, disappears on the last night of their family vacation at a resort on the Caribbean island of Saint X. Several days later, Alison’s body is found, and two local men --- employees at the resort --- are arrested. But the evidence is slim, and the men are soon released. Years later, Claire is living and working in New York City when a brief but fateful encounter brings her together with Clive Richardson, one of the men originally suspected of murdering her sister. It is a moment that sets Claire on an obsessive pursuit of the truth --- not only to find out what happened the night of Alison’s death but also to answer the elusive question: Who exactly was her sister?