Four young members of an English family, two brothers and two sisters, go for a walk to the sea, where they accidentally learn about the existence of real mermaids. One of these creatures is held in captivity in a circus, and the children decide to rescue the captive at any cost. This act will open the way for them to the underwater kingdom, full of secrets and magic. However, this is where their adventures are just beginning! Want to know the details? Go on an underwater journey with the characters of the book.