The story begins with a historical account of the world's creation by five gods who, out of loneliness, created five races: humans, elves, beastfolk, angels, and dragons. Their peaceful existence was shattered when the immense power and negative emotions generated during creation birthed a Demon God. This Demon God, possessing the power of negation but a kind heart, also created five races, mirroring the gods. However, these creations were corrupted by injustice, appearing different from the original races. Humans, fearing these new beings, refused to acknowledge them, leading the Demon God to name them: demons, dark elves, monsters, devils, and undead dragons. Humans then oppressed these newly created races.
Grieved by the suffering of his creations, the Demon God bestowed upon them the power to defend themselves. However, this immense exertion caused him to vanish, with a prophecy of his revival in a thousand years. This event marked the beginning of an eternal enmity between demons and humans, a truth now lost to history.
The narrative then shifts to Akimoto Akira, a 16-year-old high school student, on his way to school. While waiting at a crosswalk, he witnesses a boy chasing a soccer ball ignore a red light and run into the path of an oncoming bus. Despite his fear and the inaction of bystanders, Akira's body moves on its own, pushing the boy to safety and placing himself in the bus's path. Instead of impact, he finds himself in a large, dark room with six pillars and a purple magic circle. He suspects he's been summoned to another world.
Opening a door, Akira encounters a maid with small horns who is initially hostile, identifying the place as the Demon God's residence and not for humans. Akira explains he was summoned, which further agitates the maid, who insists the magic circle is for the Demon God's revival. When Akira points out she saw him emerge from it, she becomes suspicious and asks to see his status. Guided by the maid, Akira learns to access his status, revealing his name is now Diablo, his race is God (Human), and he possesses unique skills like "Demon God Transformation," "Supply," and "Demand," along with resistances to physical and magical attacks. He also holds titles such as "Demon God," "Creator," "Otherworlder," and "Reincarnator."
Diablo, still processing his new identity, is informed by the maid that he is indeed the Demon God. He is then introduced to Ridino, a butler-like figure who, along with many other demons, kneels before him, expressing honor at his return after a thousand years. Ridino explains they are in the Demon God's temple at the heart of the demon territory. He confirms Diablo's identity as the Demon God, as only the Demon God can enter the temple's core. Ridino also explains that Diablo has reincarnated from his previous world and likely cannot return.
Their conversation is interrupted by news of a hero's invasion. Ridino explains that heroes are humans and enemies of demons. Lucifer, a handsome young man in a black suit, appears and confirms the hero must be killed. Diablo is hesitant to kill humans, but Ridino and Lucifer explain that if the hero isn't killed, the demons will die. Reluctantly, Diablo agrees to the plan, requesting they try to minimize casualties and that he will accompany them.
They travel to the battlefield, where Diablo witnesses a hero single-handedly overpowering demons. This sight enrages him, triggering a voice in his head identifying itself as "Sati," a support program for the Demon God. Sati activates Diablo's unique skill "Demand," granting him "Flight Magic," "Elemental Magic," and "Enhancement Magic." He then confronts the hero, who dismisses him as a "lesser demon" due to his hidden true status. Sati reveals Diablo's true status is hidden from others, and his displayed status is that of a Lesser Demon.
Diablo, now with his true powers, uses "Ruin of Darkness" and "Gluttony of Darkness," skills he inexplicably possesses. The hero recognizes "Gluttony of Darkness" as a top-tier skill of the Demon King Belzebub, questioning how a "nobody" can use it. Sati explains the skill was recreated using elemental and enhancement magic, with "materials" found. The hero's companions arrive to help, but Lucifer and Ridino easily defeat them using "Wave of Arrogance" and "Gravity Manipulation." Diablo then fully unleashes "Gluttony of Darkness," consuming the hero.
Witnessing the hero's demise, Diablo feels an inexplicable sadness, recalling fragmented memories of a peaceful past shattered by war. He resolves to end the conflict between demons and humans. He asks Ridino and Lucifer to accompany him on a journey to understand the world. He then uses Sati to change his status to "Akira," a human with lower stats, to travel incognito.
Akira and his new companion, Cell (the former maid), are given a mapping stone by Ridino. They learn that Cell possesses a "Hide Skill" and can manipulate her status, making her suitable for accompanying Akira. They set off for a human village, but after three hours of walking, Akira uses Sati to acquire "Space Magic," allowing him to instantly teleport himself and Cell to the village outskirts. Cell transforms her appearance to that of a human village girl.
Upon entering the village, they are stopped by guards who check their statuses. Akira's human status is accepted, and Cell's transformed status is also approved. They receive a village map and are advised to find lodging. Akira realizes he has no money, but Cell produces coins given to her by Ridino, explaining they were the Demon God's money from his previous life. They check into an inn, where they are given a room with only one bed, leading to an awkward situation where they decide to share it.
The next morning, Akira and Cell go shopping for clothes. Akira buys several outfits, and Cell chooses a white dress and robe. Akira purchases them for her, revealing the innkeeper's perception of them as a couple. They then head to a weapon shop, but their plans are interrupted by a demon attack on the town. Akira, now with the title "Adjuster," rushes to the town center.
He confronts the attacking demon, "Jako," who reveals his intention was to sacrifice 1000 humans to revive the Demon God. Akira, angered by this deception, uses his "Demon God Aura" to subdue Jako. Sati reveals that the true Demon God is sealed within a crystal in a cave connected to the village underground. They find the crystal, and an angel named Lia appears, guarding it. Lia claims the sealed entity is her master, the true Demon God. Sati reveals the sealed body is the Demon God's previous physical form, sealed by Lia to allow him to reincarnate and prevent future conflict.
Akira, realizing he is the reincarnated Demon God, uses his powers to break the seal. The crystal shatters, releasing a black-haired young man, which is implied to be his previous body. Lia recognizes him and pledges her loyalty. Akira, now with partial memories restored, decides to continue his journey to change the world, accompanied by Cell and Lia.
