Demon King Ahriman, once the undisputed ruler of the demon realm and conqueror of countless worlds, has suddenly halted his campaigns. His loyal aide Jahi discovers the shocking reason for this pause: the Demon King is deeply troubled because his young daughter, Doux, is simply too kindhearted. Determined to mold the princess into a ruthless heir worthy of the demon throne, Jahi takes charge of Doux's training.

The story follows the comedic and heartwarming daily life of the demon castle as Jahi subjects Doux to one trial after another intended to awaken cruelty and evil tendencies. However, each attempt backfires spectacularly. Doux consistently misunderstands the assignments in the most positive way possible, solving every challenge through the power of kindness, empathy, and her overwhelming cuteness. Whether ordered to oppress humans or use dark magic, she instead heals the sick, befriends enemies, and brings comfort to the lonely.

The main cast includes Doux herself, a small demon child with a gentle soul who loves helping others; her devoted father Ahriman, a fearsome Demon King who secretly dotes on his daughter; and Jahi, the long-suffering aide whose serious training attempts are continually thwarted by Doux's pure heart. Other characters include Sati, a youthful succubus who teaches pranks, and Melina and Mayu, who round out the demon realm inhabitants. The series takes place in a fantasy world where demons, humans, and angels exist in conflict, though Doux's kindness begins to bridge these divides.

Notable narrative arcs include Doux's early training episodes where she turns punishment into play, her encounters with human slaves that lead to unexpected friendships rather than oppression, and her adoption of a small lizard that becomes a dragon companion. The series introduces the Pairika seduction witches, demon hunters, and even multiple versions of Santa Claus transformed from prankster demons. A significant development occurs late in the series when the origins of Doux's exceptional kindness are revealed, hinting at deeper connections to an entity named Dahaaka and raising questions about whether her goodness is innate or imposed. The story explores whether a demon raised with love can truly become evil, and whether kindness might be more powerful than cruelty in ruling the demon realm.