Second Lieutenant Yuu Mononobe has been sent to Midgard to attend the all-girls school where his sister Mitsuki is the student council president. The school is specifically for people with D powers, who are needed to fight Dragons, and both Mitsuki and Yuu fit the criteria. On his arrival, he meets and befriends another classmate named Iris Freyja, who is having difficulties with her D-related schoolwork. They will do their best to succeed in this dangerous world.