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. The next day, Yuu is placed in Brynhildr Class, where he faces opposition from the girls and is forced to perform dark matter transmutation to prove his status as a D. While watching a practice drill alongside Haruka Shinomiya, the teacher of Brynhildr Class, Yuu is informed that Dragons seek Ds as their mates and that a D's dragon mark changes color when she is targeted by a Dragon, transforming into the same kind of Dragon upon contact. Following Iris' failure at transmutation, Yuu has a chat with her, where she explains to him her reason for fighting against Dragons. However, their discussion is interrupted when an emergency alert sounds, announcing 'White' Leviathan's approach, while at the same time, Iris' dragon mark starts changing color.