The plotline refers to a person attending to the main crew's school, a regular human that holds an intense love for myths and magic. They are occasionally trying to find odd situations at their school, that might lead to the conclusion that magic is real and prove any of their theories.
In this case, the person observes one of the main crew's condition induced oddities (wariness towards water, weird powers accidentally unleashing, etc) and investigates more. After few direct sightings and more clues they come to the conclusion they found merpeople (at least one of the main crew members, if not more).
They manage to get concrete evidence and use it to blackmail the merpeople with it. But what do they really want out of this bargain? All they want is to crack the secrets of this magic, research mermaids as deep as research goes... to the point where they'd like to become one themselves.