Diana and her friends are mermaids, but they have to keep it a secret to live a normal live at high school. But, suddenly, the tension is taking over the campus: students begin to disappear, and there is a rumor that the lagoon is cursed. They suspect that behind all this mystery there may be a very dangerous kind of mermaid, but that it has been decades without causing any harm. Diana and her friends must solve the mystery and defeat the evil mermaids.