Claire is only seven years old when her college-age sister Alison vanishes from the luxury resort on the Caribbean island of Saint X on the last night of her family's vacation. Several days later Alison's body is found in a remote spot on a nearby cay, and two local men, employees at the resort, are arrested. But the evidence is slim, the timeline against it, and the men are soon released. It's national tabloid news, a lurid mystery that will go unsolved, but for Claire's family there is only the sad return home to broken lives.
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?