Lucy s friend, Morag invites her on a paddle steamer trip through the Scottish lochs with her uncle, Captain Ming.

Lucy is a special guest, for she once helped Kelpie, the magical water-horse. And though Morag doesn t believe in Kelpie and things like that, she does believe that Lucy will see the mermaid that Captain Ming loves, and be able to stop her from enticing Captain Ming away.

A rich weave of fantasy and reality, with the reader - in true Mayne style - never quite sure where one ends and the other begins.