Second Lieutenant Mononobe Yuu has been sent to Midgard, to attend the all-girl's school where his sister Mitsuki is the student council president. The school is specifically for people who have "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 Freya, who is having difficulties with her "D"-related schoolwork. They will do their best to succeed in this dangerous world.

(Source: MangaHelpers)