The story is set in a world where humans with supernatural powers known as Blazers can materialize a Device, weapons made through a person's soul. Ikki Kurogane, dubbed the Failure Knight by everyone around him, encounters a half-naked girl, Stella Vermillion, as she changes in his room. This encounter changes Ikki's life as he strives to become a real Mage Knight by reaching the peak of the Seven Star Sword Art Festival, an annual event held by the seven Mage Knight Academies in Japan to determine the strongest Apprentice Knight between the schools. The winner of the festival would be known as the Seven Stars Sword Kings. The Seven Star Sword Art Festival consists of actual combat and competitors may die in the festival.

In a time and place where one's soul can be morphed into a weapon, there are modern-day magicians called mage-knights. Although Ikki Kurogane is a student at an institution that trains mage-knights, he has no particular talent in magic and is labeled the "Failed Knight" or "Worst One." Getting way less than average marks in the scorings, he was forced to repeat a year. But with the arrival of a new head of the institution, a new rule was created: knights whose abilities are compatible, as decided by the board, must share rooms and attend practice and training together throughout their school years to bring up their abilities to the max. It was a rule to implement the absolute verdict of ability.

Ikki's roommate, Stella Vermillion, turns out to be a princess of a foreign country. Stella is an A-Rank knight, the type of genius in magic who only appears once a decade. When Ikki walked in on her while she was changing her clothes, it caused a huge misunderstanding, which eventually ended up in a duel between the two of them. The punishment for the loser is eternal submission to the winner.

Forced to live in the same room and practice magic together throughout all their school years, how will Stella and Ikki's relationship evolve?