Onodera Goushi is a high-schooler who isn't much noticed in other people's daily life. In a sense, his life feels like it would be the same if he wasn't around. Then one day while returning from school, he finds himself in a life and death situation where a strange girl named Honoka saves him by sending him to another world. He then gets sent to the "Demon Nation" where many non-humans live and where he is able to meet the Ma-Ou (Demon King) who is... an anime otaku?!?!

In the midst of a terrorist attack that kills all his classmates,Tohru Kuroe knows the chances of seeing yet another woman die makes Tohru pick up an abandoned gun - The beast awakens; taking over. When Tohru comes to his senses, he finds he is now a killer. Finding out he was genetically engineered to be a killer, his codename is D-17, Tohru continues his 'normal' life amidst challengers threatening his life and those around him.