Seven years prior, high school student Kiriya Seiichiro was unwillingly summoned to another world by a "crazy nation" that weaponized humans. After a year of secretly accumulating power, he escaped, eventually destroying the nation and sadly eliminating his weaponized compatriots. For five years, he lived with ancient artifacts called Makina Dolls, including his loyal aide Aine. His ultimate goal was to return to Earth.
Seiichiro developed a return transfer magic circle, but the immense magic required meant he could only return to the exact time and place of his original summoning. This posed a problem: seven years in the other world had matured him physically and mentally to the equivalent of a 22-year-old. To blend in, he used seven "lifestones" to de-age himself by seven years, returning to a 15-year-old body, albeit one with an unusually sharp gaze and lean musculature. After having modern clothes tailored (as sneakers couldn't be replicated), he activated the transfer array in his underground base.
He reappeared on a sidewalk near a convenience store in Japan, unnoticed. Confirming the date on a newspaper, he found it was indeed the day he was summoned seven years ago. A wave of nostalgia washed over him as he read manga, remembering the stories. On his way home, he encountered three rude thugs blocking the street. Annoyed, he led them to a secluded alley and effortlessly incapacitated them, giving them a "meaningless educational guidance."
Returning home, he was greeted by his mother and later his younger sister, Midori. He quickly changed into his old clothes, stashing his new ones in his Item Box, having confirmed his magic and skills still worked on Earth. During dinner, the taste of home-cooked food brought tears to his eyes, and Midori noticed his changed demeanor and sharp gaze. He fabricated a story about eye strain and promised to get glasses. The next morning, he acquired cheap glasses, which he secretly modified using alchemy and mithril to be non-prescription and incorporate a small, long-lasting concealment magic array to soften his intense stare.
On the day of his high school entrance ceremony, Seiichiro attended Kashiwagi High, aiming to graduate for his parents' sake despite his immense wealth. He was assigned to Class 2, securing a window seat in the back. During self-introductions, he met classmates like Akiyama Yoshiki, a famous soccer player; Nakagawa Haruna, the new student representative from a prestigious school who gave a terse introduction; and Murata Hideo, an otaku from his old middle school whom Seiichiro didn't remember.
After homeroom, Seiichiro intervened when two "bad-eyed" students, Mob A (from an influential family) and Mob B, tried to drag a reluctant Murata away. Seiichiro insisted Murata stand up for himself, and when Murata finally refused, Seiichiro offered himself as a target. Mob A and B punched him, but their fists shattered against his unyielding face, leaving them with complex fractures and unconscious. Seiichiro and Murata were taken to the staff room, where their teacher, Ms. Yamada, accepted Seiichiro's explanation as an "unfortunate accident" but warned him against such recklessness. Murata confided that the two had bullied him for years, and Seiichiro promised to protect him.
Back home, Seiichiro installed a mithril transfer array in his room, linked to his base in the other world, to allow low-cost travel and synchronize time flow. He also set up mana collectors. Returning to his base, he discovered that three months had passed in the other world while only a few days had passed on Earth. Aine informed him that the Agataria Army, detecting his mana signature's disappearance, had invaded his territory. Enraged by this "sneak attack," Seiichiro ordered a punitive counter-attack: the complete destruction of Agataria's capital royal castle. He also dispatched Fyur, another Makina Doll, to "play" with a self-proclaimed "Hero Party" approaching his defenses, instructing her not to harm them.
Meanwhile, Agataria's Lieutenant General Eisen, who had reluctantly led the invasion, foresaw his kingdom's doom, understanding the true power of the "Demon King Kiriya" (Seiichiro). He dismissed his arrogant subordinate and ordered a full retreat, refusing to sacrifice a rearguard. The "Hero Party" struggled against Seiichiro's golems before Fyur arrived, effortlessly breaking their "holy" weapons and condescendingly dismissing them. Simultaneously, Agataria's Royal Guard Wyvern Squadron was decimated by Seiichiro's airships using invisible "Tele-Explosion" attacks. The airships then proceeded to bomb Agataria's capital royal castle, first with hard bombs, then with incendiary bombs, leaving it a smoking ruin. Seiichiro, having returned to a semblance of normal life, was now forced to unleash his overwhelming power to protect his domain, demonstrating his ruthless side when provoked.
