Twenty-three years ago, Christina Tolmie vanished without trace from northern France, leaving her young daughters Kate and Flora orphaned and alone.

Now Flora is also missing. In desperation, Kate searches her Edinburgh house, and finds a piece of notepaper with just one name: Cal McGill.

Cal is a so-called sea detective, an expert on the winds and the tides, and consequently an exceptionally gifted finder of lost things - and lost people.

Kate hopes that Cal might not only find her sister, but also unlock the mystery that has overshadowed both women's lives: what happened to their beloved mother all those years before?

Unfortunately, Cal doesn't think he can help. But that's only because he hasn't yet realized that the dark undercurrents of the case will ultimately lead him back dangerously close to home . . .