Hiroki Katayama is your average high school kid. He moved to the undersea city Blue Genesis because of his foster dad, his uncle Shigeru's, job.
Right after he got there, he could tell this place wasn't exactly normal. There were all these rumors, like ghosts showing up, or people getting attacked by monsters called Nightmares. Everyone acted like it was a joke, but still...
The night they moved in, some weird creature showed up right in front of him. It called itself Navi and said if they lived together, it could awaken his psychic powers. Navi told him there's some huge disaster happening in the real world right now, and if they don't stop it, everything's going down for good.
It sounded like something that had nothing to do with him, but just like the rumors said, Nightmares really were wandering around Blue Genesis. So even though he didn't want any part of this mess, he started looking for other espers just like Navi told him to.

[Translated from the official website]