After both his parents died in an accident, Tsugawa Tatsuya is now left with millions of heritage he can't use. In the weeks after, he's still mourning and thinking about his father's last words, "Tsugawa family's man must be a man of great vessel". But Tatsuya is not confident he can fulfill his father's last wish.
One day, he met Miyazaki Seine, a strange girl who seems to enjoy being bullied. Tatsuya believes he can help her even though his friends only see her as a masochist pervert, and Seine herself told him to mind his own business.
Seine hunts creatures of the sea, a kind of parasitic bug that controls humans and influences them to do hateful things. She possesses an immortal body and a beast called AiON that's used to lure and devour the bugs from inside the human host. She and the bugs naturally have an impulse to kill each other on contact.
Even so, Tatsuya still can't just leave her alone, he wants to help her.