Mari Prothero lives a simple life on the Welsh coast with her fisherman father, Daffyd Prothero, who always comes home with a catch. For her whole life, it has just been the two of them. Well, them and the small fairy folk she can see around her house. Their world begins to be turned upside down when a constable comes to their quiet town of Clogwyn and starts to become a nuisance, prying into business that is not his, particularly when it comes to the Protheros. After encountering him and making him go away, Mari encounters a small water spirit, who tells her to bargain in the future. It makes sense when her father reveals to her that she must marry a Selch and then turn over her firstborn child to the Selch clan, to keep the Prothero luck going. At first, Mari is beside herself, but then remembers the small water fairy's words. The spirit appears again and informs her she has magic in her blood. Emboldened, she confronts the Selch chief and demands him to send her suitors and a teacher to school her in the art of Water Magic. The chief rails at her, but in the end brings four suitors, Trefor, Siarl, Rhodri and Mabon, and a teacher, Idwal. Her lessons begin in earnest, learning she is a Water Master and also spending time as a seal, and begins to fall for her teacher Idwal, even as the suitors begin to leave. In London, Sarah and Nan have returned from walking through the wildernesses of Africa. However, the two young ladies do not know what to do with themselves. They try teaching at Mem'sab's school, but find that they are not teachers. Despairing of what to do with themselves, their friend and benefactor Lord Alderscroft suggests the two of them to travel to Wales to seek a Water Master that is located somewhere in the country. The two immediately accept, and with Neville and Grey in tow, travel to Clogwyn. The two girls set themselves up in a comfortable cottage and come up with a ruse that the two of them are sisters. With the help of their old friend Puck, they discover the truth about Mari. Before they can do anything about it, they are set upon by the constable, who is then driven away by Puck in dog form and a sassy Neville. Upon encountering Mari, the three girls become fast friends, and as the suitors continue to leave, one of them, Rhodri, ends up falling for Nan. Mari and Idwal fall head over heelsin love, and state their intent to get married. With Rhodri remaining and pretending that he is still courting Mari, all is well until the Selch chief, Gethin, blows into the cottage in a rage, accusing Mari and Idwal with treachery and for stealing his Druid. Mari rebuffs him, informing him that he did not specify that she could not fall in love with her teacher, which Idwal reiterates. Gethin is taken aback, then spitefully reminds her of the Prothero luck and the promise her ancestors made. He storms out of the cottage, leaving the group behind. Soon after, Mari and her newfound friends cook up a scheme by which she is wed forcibly to Idwal, and the two are married and soon Mari is happily pregnant. Nan and Sarah depart for a time and must obtain winter clothing and goods before returning to the cottage just in time for the birth of Mari's twin boys. As soon as the boys are born though, Gethin storms in and seizes the boys and enchants Idwal into returning to the sea with him. Mari rages and weeps, and things get worse after Nan and Sarah depart to seek help and when Constable Ewynnog returns and arrests her father on charges of murder of her husband and children. Upon being alerted by Puck, Nan and Sarah rush back to the cottage just in time to stop Mari from killing the Constable. With the help of Puck, a few ghosts, and the local Water and Earth Elementals, the three manage to free Daffyd. With Puck taking him, the three women make their way to the coast, where they meet Rhodri. Rhodri takes Mari to the group of Selch, and Mari challenges Gethin to allow her to get Idwal and her children back. She barters with her life, and faces three challenges. The first, the Test of Courage, has her holding onto Idwal while he changes forms, including a serpent, burning log, and ice. She succeeds, and then is given the Test of Wisdom. She goes toe to toe with Gethin and answers all of his riddles, even when Gethin begins to lose his cool. The final test, the Test of Love, is where she must pick Idwal and her babes from a crowd of Selch folk that all look the same. At first she despairs, but puts on her sealskin, and in an instant is able to find her babes and Idwal soon after. Gethin flies into a rage, and banishes Idwal and Mari from the clan, much to the disapproval of the others, and withdraws the Prothero luck. Idwal responds that they do not need it, and they return to the cottage. None too soon when the constable shows up, intent on arresting Daffyd, and bringing some fellow officers with him. But when they arrive and find the babes and Idwal, the constable is shuffled out of Clogwyn for good. The story ends with Idwal and Mari standing on the coast and gazing out to sea.
