Nia is a young mermyd, supremely qualified to become an Avatar of the city of Atlantis, and awaiting the day when she is chosen to compete in the trials to become an Avatar. When her less qualified cousin is chosen to represent the Bluefin clan in the trials, she begins to unravel the dark secrets kept by her family and the Avatars of Atlantis.
Though I haven't read much in the way of mermaid fantasy, I did enjoy the unique twists on their civilization that seem like they would make the mermyds of Dakley's world stand out among the rest. Some of her mermyds have legs, others have the characteristic fins, and those of the Sealion clan even has fur in lieu of scales on their fins (which I found exceptionally clever!).
The big "twist" doesn't happen until the last two chapters of the book, which leaves just enough time for a huge cliffhanger, so I definitely wouldn't recommend it to anyone looking for a stand alone book.