Kisaragi Ryuuji is a normal high school boy living a peaceful life, which is turned into an adventure when his second cousin Eriko suddenly returns from overseas. He gets recruited into her organisation, the Seven Tails, in order to help her find artefacts called Lost Precious. Ryuuji and Eriko manage to seize a relic box from a black broker named Fang. Inside the box they find a girl whom Ryuuji names Rose because of the rose like pattern on her left hand. When it turns out that Rose is in fact a Red Dragon, Ryuuji decides to protect her from the black organisation using his powers as a level 10 Breaker.