In a time and place where one's soul can be morphed into a weapon, there are modern day magicians called Mage Knights. Although Kurogane Ikki is a student at an institution that trains Mage Knights, he has no particular talent in magic and is labeled the Failure Knight or the Worst One. Getting way less than average marks, he is forced to repeat a year. But with the arrival of a new head of the institution, a new rule is created: knights whose abilities are compatible, as decided by the board, must share rooms and train together throughout their school years to maximize their abilities.
Ikki's roommate turns out to be Stella Vermillion - a princess of a foreign country and a rank A knight: the type of genius in magic who only appears once a decade. When Ikki walks into the room while she is changing her clothes, it causes a huge misunderstanding that eventually leads to a duel between the two. The punishment for the losing side is the Eternal Submission to the winner.
Forced to live the same room and practice magic together throughout their school years, how will Stella and Ikki's relationship evolve?