Hiyori Yameda is a strong-willed high school girl whose forehead bears a permanent scar from a past gang fight involving her boyfriend, Yuuto Inudou. Yuuto, once the leader of a notorious gang, carries heavy guilt over the injury and becomes overly protective of Hiyori. To keep her safe, he orders his younger brother, Ryuuma Inudou, to act as her personal watchdog. Ryuuma is known as an uncontrollable mad dog among local delinquents, a hotheaded and violent youth who resents the task. Despite his refusal to be gentle, he always appears in Hiyori’s moments of danger, often biting her on the neck in a possessive gesture that leaves her confused and flustered. Hiyori remains devoted to Yuuto, but Ryuuma’s rough brand of protection and his hidden feelings begin to shake her heart. A love triangle forms as Yuuto’s past and Ryuuma’s growing attachment pull the three into a cycle of danger and emotional turmoil.

Yuuto and Hiyori’s relationship deepens, culminating in a night together on her birthday. However, Yuuto is secretly injured while saving a child from a truck, and he dies the next morning from his injuries. Hiyori is devastated. Three years later, she still mourns Yuuto while Ryuuma, now determined to protect her in his brother’s place, confesses his love. Hiyori struggles with her loyalty to Yuuto’s memory and her evolving feelings for Ryuuma. A rival girl, Yumea, appears and schemes to separate them, but Ryuuma ultimately rescues Hiyori. After a reluctant date, Hiyori realizes she has fallen for Ryuuma. The story closes with Hiyori and Ryuuma starting a new relationship, overcoming the shadow of Yuuto’s loss.