In an alternate Earth, humans called "Blazers" have supernatural abilities. These Blazers can materialize weapons known as "Device" which are made through a person's soul.
At Hagun Academy (破軍学園, Hagun Gakuen), Blazers are selected as representatives for the Seven Star Sword Art Festival, an annual tournament event held by the seven Mage Knight Academies in Japan to determine the strongest Apprentice Knight.
Hagun's performance ranking in the festival is falling and the academy's director, Kurono Shinguji, is determined to find a solution to the problem. Ikki Kurogane is the academy's F-Rank Blazer and is considered "The Worst One" for his low magical abilities, but Stella Vermillion, the princess of the European country Vermillion, is one of the top A-Rank Blazers.
On Stella's first day at Hagun Academy, she is arranged to share a room with Ikki as a transfer student of the academy. When Ikki inadvertently discovers Stella half-dressed, he is challenged to duel where the loser has to be obedient to the winner for life.
She ends up losing the duel, but they agree to become roommates as Ikki's one and only merciful desire. The series follows their adventures as they train to qualify as the school's representatives for the festival.