The story follows Youki, a reincarnated individual who finds himself in the body of a demon in the middle-to-late stages of the Demon King's castle. Possessing cheat-like abilities from his previous life as a human named Segawa Haruki, Youki's primary goal is to prevent world chaos by ensuring the Hero doesn't fail. He defeats the Hero's party, consisting of the Hero, a Mage, a Swordsman, and a Cleric, but instead of eliminating them, he heals them and orders his subordinates to abandon them near a village. This decision stems from his lingering human morality and a particular attraction to the Cleric, Cecilia.
Over the next month, the Hero's party repeatedly challenges Youki, only to be defeated each time. This cycle frustrates the Demon King and his executives, who believe the Hero's party is weak, unaware of Youki's overwhelming power. Youki, however, realizes his interference is preventing the Hero's party from growing strong enough to defeat the Demon King and save the world. He decides to allow them passage, particularly intrigued by Cecilia, whom he finds kind and considerate.
Youki proposes a deal to the Hero: he will allow the party to pass if they leave Cecilia behind. The Hero initially refuses, but the Mage, motivated by rivalry, persuades him to accept. The Hero's party departs, leaving Cecilia with Youki. Youki, confessing his love, is rejected by Cecilia. Devastated, he contemplates death, but Cecilia, despite being his enemy, shows concern. She questions his unusual behavior for a demon, and Youki explains his human past and his reluctance to harm humans. He reveals his desire to live a normal life and his inability to serve the Demon King's destructive agenda.
Cecilia, understanding his plight, proposes he join forces with humans. She suggests he use his power for their benefit and offers to help him integrate into human society. This leads to Youki leaving the Demon King's castle, presumed dead by the Hero's party. He begins a new life, joining a guild and working as an adventurer. He diligently takes on quests, aiming to earn money and improve his standing, with the ultimate goal of confessing to Cecilia again.
During his time at the guild, Youki befriends Raven, the Swordsman from the Hero's party, who reveals his own insecurities about his voice. Youki also meets Cecilia's mother, Seria, who, after initially suspecting Youki of having ulterior motives, approves of him and his relationship with Cecilia. Seria reveals that the Hero is still pursuing Cecilia, but reassures Youki that he has her blessing.
Youki's former subordinates, Duke, Happiness, and Seek, who survived the Demon King's downfall, find him. They express their concern and loyalty, and Youki learns that his subordinates escaped the castle after the Demon King's defeat. He explains his current situation, including his decision to abandon his demon form and his unrequited love for Cecilia. His subordinates are shocked by his confession.
Youki continues his adventuring career, aiming to become strong enough to win Cecilia's heart. He faces new challenges, including encountering his former subordinates and navigating the complex social dynamics of this world.