Seoul is plagued by the 'Beast Disease,' a disaster that turns humans into beasts. Lire, a boy with unusually sharp instincts, lives there, caring for his sick younger brother all by himself. Then one day, a mysterious incident occurs at the hospital. Trying to protect his only family, Lire awakens on the boundary between human and beast. "I promise—I will definitely find you back." "Even if I have to become a beast."