A series of big storms flood the sea town where Camila lives and she finds a tiny, newborn mermaid in a puddle of water. Camila adopts her and names her Mermary. She tells no one, especially her mother, a marine biologist who would take the mermaid away for research and study. Camila is extremely shy and the mermaid becomes her only friend. She reads everything she can about mermaids and earns the nickname "The Mermaid Girl" from her classmates. Before long, rumors spread of a mermaid in the lake. Now Camila has to figure out how to protect Mermary not only from scientists but from people who want to do more than just catch a glimpse of a mermaid.