Mari Prothero grew up in a tiny fishing village on the west coast of Wales where she lived alone with her father, Daffyd, a master fisherman. Although the sea had never harmed her father, Mari still worried, for her mother and older brother had been drowned by a rogue wave when she was just an infant. Her father was all she had left, and Mari was not blind to the possibility of losing him.

Mari and her father kept to themselves in their village. It would not be wise to call attention to how much more prosperous the Protheros were than any other fisherman's family; Daffyd was out in every storm, pulling in fish where no other man could, and his fish were always the fattest, the tastiest. But their unusual good fortune was not the only reason for their reclusive behavior.

For as long as she could remember, Mari had seen things--things that shouldn't, that couldn't be real: tiny manlike creatures that were mischievous and wore only seaweed, and tiny beings that spoke to her kindly but seemed to be made of water. When she was a child, she had asked her father about these creatures, but Daffyd was so upset by these conversations that Mari had given up trying to talk to him.

But on the morning of her eighteenth birthday, her father finally told her the great secret of the Prothero Luck--and what they must do to keep it. Her mother and brother were not truly dead, but neither were they human. They were shape-shifters of the sea, the half-human, half-seal people known as the Selch, and they had returned to their watery domain. Since ancient times, the Prothero family had kept a covenant with these magical creatures. In exchange for protection on the water, a member of the Prothero family would take a Selch to marry, only to relinquish their spouse and one of their children to the sea after a brief number of years.

Now Mari's time had come, and her father told her that she must abide by her family's ancient magical compact, or face dire consequences. But Mari was not without protectors: for the tiny creatures she had seen her whole life, which she now knew were real, counseled her to "bargain" with her mother's people. While in faraway London, Lord Alderscroft, the head of the Elemental Masters, was aware that a powerful but untrained Water Master on the far coast of Wales was being threatened by some of the Elemental Elder Spirits, and he had dispatched some very unique champions to come to Mari's aid....