Kate and Flora have always been haunted by a mystery - their mother, Christine, vanished without trace when they were children. But now Kate has a more urgent problem: Flora has disappeared too. In desperation, she searches Flora's house, and finds a scrap of paper with a name scribbled on it: Cal McGill. Cal is a 'sea detective': an expert in the winds and the tides, and consequently adept at finding lost things - and lost people. Can Cal find Flora? And might he even know the secret of what happened to their mother, all those years ago . . . ?