Following the tragic incident that claimed the lives of both his parents, Tsugawa Tatsuya finds himself in possession of millions in inheritance that he's unable to utilize. As he continues to mourn and ponder over his father's final words, "Tsugawa family's man must be a man of great vessel," Tatsuya struggles to muster the confidence to fulfill his father's wish.
One fateful day, he crosses paths with a peculiar girl named Miyazaki Seine, who appears to relish in being bullied. Though Tatsuya is eager to extend a helping hand, his friends dismiss her as a masochistic pervert, and Seine herself rebuffs his offer of assistance.
Seine's mission involves hunting down sea creatures - parasitic bugs that manipulate humans and compel them to commit malicious deeds. With an invincible body and a creature named AiON that lures and devours the bugs from within their host, Seine shares an instinctive urge to eliminate her foes.
Despite the immense risks involved, Tatsuya remains resolute in his determination to aid Seine in her exploits.
