Myra and Kassy Cooper have discovered a magic library. An evil witch is taking characters out of the books and making them forget who they are.
Myra and Kassy have to save the characters before their stories disappear forever.
In this book, Myra and Kassy are swept out to sea on a small rowing boat and they meet a mermaid. The mermaid knows nothing about the evil witch, but her parents do and realise that everyone is on danger.
Despite getting away from danger, the mermaids are soon under the witch's spell and find themselves trapped in a tower.
It's up to Myra and Kassy to save them. This is made more difficult when an angry sea serpent comes after them.