A former overworked corporate programmer dies from overwork and reincarnates as a baby in a demon king's castle, surrounded by eight of the most powerful demon lords in the world. Initially at risk of being dissected to study the weaknesses of heroes, the infant is spared when the demon king Hades argues that the child possesses intelligence and potential, and that raising him as their own would yield a far greater asset. The demon lords adopt him as their hidden child and name him Aldeus, calling him Al for short.

The story takes place in a fantasy realm where magic exists but operates on rigid, traditional rules. Al grows up inside the demon king's castle, cared for by a dedicated maid named Sylvia and tutored by the various demon lords, each of whom is a master of a different combat or magical discipline. Drawing on his past life as a programmer, Al begins to develop a revolutionary system he calls Magic Code — a form of magic that functions like computer programming, allowing him to write, debug, and optimize spells with logic and syntax rather than innate talent or incantations. This system gives him a unique edge, enabling him to create custom spells, automate magical processes, and eventually rival the power of his demonic guardians.

The central conflict arises when the heroes of the human world, seeking to destroy the demon king's domain, begin to target Al and those close to him. An entity called the Hero's Party attacks the castle, forcing Sylvia into a desperate battle where she nearly dies. Al intervenes with his Magic Code, saving her and demonstrating that his programming-based magic can stand against even the most overwhelming physical force. Later, a girl named Liz, who possesses a powerful magical eye, is captured by a cunning hero who attempts to extract her eye. During the extraction, Liz glimpses the hero's past-life memories and comes to understand her own powers. She chooses to destroy her own eye rather than let it be stolen, an act that draws the attention of a hidden shadow conspiring in the background.

As the story progresses, Al's Magic Code evolves from a curiosity into a decisive weapon. He faces assassins, infiltrators, and political schemes from both human forces and internal demon factions who distrust his half-human origin. The narrative balances action, magical research and development, and the found-family dynamic between Al and the demon lords who raised him. Notable arcs include the battle to defend the castle against the Hero's Party, the rescue and recovery of Liz, and the investigation into the mysterious force manipulating events from behind the scenes. The series concludes with Al fully establishing himself as a new kind of ruler who blends the demon lords' ancient power with his own logic-based magical system, securing peace for his adoptive family.