To avenge his younger sister's death, Tooru Kokonoe enrolls in the prestigious Koryo Academy, a high school focused on training students to wield Blaze—individualized weapons that manifest from one's soul. Although a student's future is typically determined by their weapon's form, Tooru is an Irregular, with a Blaze that appears as a shield rather than a melee weapon. How can Tooru possibly learn to use his defensive abilities to fight effectively? That is when the Duo system comes into play, a program pairing students as battle mates, classmates, and roommates. Tooru is paired with Julie Sigtuna, a silver-haired girl from Scandinavia who possesses unprecedented power and shares his thirst for revenge.