A long time ago when humans were at war with one another, the demon king rose in power and took over the land. Humankind had no choice but to unite and fight as one against the demons. As a result, the demon king was successfully overthrown.
Eighteen years later, a young man called Rein and his childhood friend Ciel are working their way up to become prestigious imperial knights. One day, when the two happen to walk close to a cave, they find a female demon who greets them with an eerie grin and says, "Welcome back, my king."