It won the Nestlé Children's Book Prize Silver Award and was longlisted for the Carnegie Medal.

Sapphire, Conor, and their mother have moved to St Pirans with Roger, leaving behind their cottage by the sea, where their dad disappeared two years ago.

Sapphire skips school and takes her dog to see Granny Carne, a magical old lady.

She phones home from Granny Carne's and tells her mother she's staying over and won't be back till morning.

He tells her that the tide knot is loose, but he can't stay long and leaves shortly after.

Shortly, after they return, the tide knot loosens and nearly the entire town floods.

Elvira heals Sapphire and together the four help Saldowr, the wisest of the mer and the keeper of the tide knot, to put the broken pieces back together to restore the tide knot.