Simon Watson, a young librarian, lives alone in a house that is slowly crumbling toward the Long Island Sound. His parents are long dead. His mother, a circus mermaid who made her living by holding her breath, drowned in the very water his house overlooks. His younger sister, Enola, ran off six years ago and now reads tarot cards for a traveling carnival.
One June day, an old book arrives on Simon's doorstep, sent by an antiquarian bookseller who purchased it on speculation. Fragile and water damaged, the book is a log from the owner of a traveling carnival in the 1700s, who reports strange and magical things, including the drowning death of a circus mermaid. Since then, generations of "mermaids" in Simon's family have drowned--always on July 24, which is only weeks away.
As his friend Alice looks on with alarm, Simon becomes increasingly worried about his sister. Could there be a curse on Simon's family? What does it have to do with the book, and can he get to the heart of the mystery in time to save Enola?