With the sudden appearance of the monsters that came to be called Dragons, the world completely changed. Before long, supernatural girls with the power of the Dragons, called D, were born within the human race. The secret existence of the only male D, Yuu Mononobe, was forcefully thrown into the school where the girls of D gathered, Midgard, where he saw the naked body of one of the students, Iris Freyja. Furthermore, upon meeting his long lost sister, Mitsuki Mononobe, Yuu seems to have been enrolled into this school...!? The curtains open as the story about the boy who was supposed to become the strongest assassin and the poorly performing girl unfolds!