When Aster wakes alone on a tropical island, she has no idea what has happened, why she is there, or where to find her younger sister, Poppy. Meanwhile Sam, who once met the sisters on a plane, makes links between the mystery of their disappearance and suspicious happenings in his own life. In a stunning dual narrative, the truth unravels with devastating effect - and the answer lies in the secret underwater world surrounding the desert island, populated by the beautiful and the impossible.