Cadence Sinclair-Eastman, known as Cady, has grown up in a wealthy, prestigious family. Every summer her whole family heads out to a private island to vacation. Cady’s cousins plus a family acquaintance (The Liars) are her companions each summer. Cady looks forward to spending the summer with them until one summer when everything goes wrong. All she can remember about that summer is that she was in some sort of accident. Convinced that she had a swimming accident where she hit her head on a rock, Cady can’t understand why her mother won’t let her return to the island the following summer. Two summers later, Cady’s mother believes she might be ready to return and see her family. Everything seems normal, until Cady begins to remember little pieces of what really happened the summer of the accident.