The Demon King Ahriman once led his armies across the world with unquestioned authority; yet his campaigns come to an abrupt halt for an unexpected reason: his daughter, Doux, is far too kind. While she was born in the demon realm, she seems otherwise incapable of being cruel, as she regularly befriends enemies, comforts monsters, and treats even humans with warmth, leaving her father uncertain how a child like her could ever inherit the throne.

In a vain attempt to find a solution to this insurmountable flaw, Ahriman entrusts his loyal aide Jahi with the task of reforming Doux's disposition. Through a series of lessons and trials meant to cultivate ruthlessness, Jahi attempts to guide the young princess toward becoming a proper demon. Each effort, however, produces the opposite result, as Doux's sincerity disarms hostility and spreads unexpected goodwill wherever she goes, proving that changing the princess may be far more difficult than conquering any continent.