Absolute Duo follows the viewpoint character, Tor, who enrolls in Koryo Academy, a high school where its students battle each other with weapons known as Blaze (焔牙/ブレイズ Bureizu) as training to become future peacekeepers. Tor's ability manifests as a shield, making him an Irregular (イレギュラー iregyurā). Tor is paired with Julie Sigtuna, and must share a room with her. Both Julie and Tor are revealed to be an avenger, while Julie herself is an El-Awaken and possesses unlimited power. They both also have tragic past because their loved ones. Tor previously befriended with Imari Nagakura who failed the qualification and is enrolled in the Koryo branch academy on a vacation island and later paired with Miwa to form a duo.