Chapters 3 through 12 of the light novel detail the unexpected arrival of Sawada Yuriko, a 24-year-old former idol producer, in the Demon Realm and her subsequent partnership with the Demon King. Yuriko, who died in a traffic accident, finds herself in the Demon King's castle, where she is initially met with surprise and confusion by the imposing, black-armored Demon King.
The Demon King, despite his fearsome appearance, is revealed to be insecure and lacking in confidence. He was chosen as Demon King primarily for his imposing physique, not for any inherent leadership qualities or ambition. He struggles to inspire his subordinates and feels inadequate in his role, constantly striving to be the "best Demon King" he can be, though he admits his efforts are lacking.
Yuriko, recognizing a kindred spirit in the Demon King's struggle to fulfill his potential, is deeply moved. Her past experiences as an idol producer, where she poured her efforts into nurturing raw talent that ultimately failed to persevere, have made her empathetic to his situation. She sees in the Demon King a "gemstone" that needs polishing. Driven by a newfound purpose, she boldly declares her intention to "produce" him into a magnificent Demon King.
Their "Demon King Production Plan" begins with Yuriko helping the Demon King solidify his vision of the kind of Demon King he wants to be. She learns about the Demon Realm's ongoing conflict with the human world, the ten-year cycle of battles between the Demon King and the Hero, and the importance of the final confrontation. Based on this, Yuriko focuses on enhancing the Demon King's image and presence.
This involves designing a new, awe-inspiring and dignified set of armor, incorporating a unique icon that serves as his symbol. She also introduces the concept of merchandise and fan acquisition, creating badges of varying colors and materials for different ranks within the Demon King's army and for his supporters. Yuriko orchestrates public appearances and uses magic to resolve local disputes, all while the Demon King diligently trains to improve his skills and overcome his weaknesses.
Meanwhile, the Hero's party, consisting of a determined young Hero, a skilled magician girl, and a devout young monk, makes their way towards the Demon King's castle. They have endured a long and arduous journey, facing numerous challenges and hardships, all in preparation for their ultimate battle.
As the Hero's party finally reaches the Demon King's castle, they are met not with immediate aggression, but with a surprisingly orderly reception from the demon forces. The demons, now motivated and loyal due to the Demon King's improved leadership and the introduction of his symbolic merchandise, stand aside, allowing the Hero's party to advance.
The climax of these chapters sees the Hero's party confronting the Demon King. However, the Demon King is not focused on their strength or their mission. Instead, he is furious at their disheveled appearance and lack of "stage presence," viewing them as unworthy opponents for his "greatest performance." He berates them for their worn-out clothing and equipment, deeming them "failures" as heroes.
Overwhelmed and confused by the Demon King's bizarre outburst, the Hero and his companions are further bewildered when the Demon King calls upon Yuriko. He commands her to "produce" the Hero's party into worthy opponents for him. Yuriko, embracing this new challenge with enthusiasm, readily accepts, setting the stage for a unique "hero production" that will take place in the human world.