Sanda Ichigidai lives a quiet, strictly regulated life as a middle schooler in a future where children are a rare and precious resource, until the day his classmate Shiori Fuyumura attacks him with a kitchen knife. This startling confrontation, at the heart of the series also known by its Japanese title SANDA, unearths a supernatural lineage that the boy never suspected: he is the direct descendant of the legendary Santa Claus. The inciting ritual reveals a reality-bending twist, as Sanda finds that consuming anything red from a simple gummy to a crimson berry triggers a metamorphosis into a towering, muscular adult with incredible strength. Now, he is thrust into a bizarre and dangerous world where his very existence is a miracle, forced to balance the chaotic demands of puberty with the ancient, heavy responsibilities of a holiday long forgotten by a cynical, aging society.

The journey ahead is fraught with both external threats from a school system that hides dark secrets and an internal battle against the physical and emotional toll of his sudden transformations. Sanda must navigate complex rivalries and the weight of Shiori’s desperate mission to find her missing friend, all while facing a world that views children as icons to be controlled rather than people to be understood. Despite the absurdity of his condition and the enemies that seek to exploit his power, Sanda’s ultimate goal remains clear: he will protect his peers and reignite a sense of wonder in a dying world. Driven by a newfound resolve, he refuses to let the spirit of giving die, fighting to become a beacon of hope and a true hero for a generation that has never known a miracle.