25 years before the start of the series, massive creatures known as Dragons began appearing around the world, causing widespread destruction before vanishing. Shortly afterwards, humans with powers similar to that of Dragons were born and came to be known as D. At present day, the world's only male D, Yuu Mononobe, is transferred to Midgard and encounters a naked girl, Iris Freyja, at the beach. Iris mistakes him for an intruder and unsuccessfully tries to attack him, but then loses consciousness due to her attack backfiring and is saved by him. While the two talk, Yuu is approached by his sister, Mitsuki Mononobe, who announces to him that he will be a Midgard student henceforth.