Tamaki Amajiki used to be a normal high school man, he found himself to become a merman, before he has nightmares...
He dated with a new student in his class FlutterShy, she has same personality like his, in that case, they are almost close...
She told Tamaki, she has secrets too, she will invite him to a masquerade party, unexpectedly, it's not FlutterShy who's waiting for Tamaki...