The most powerful king in history, Grey, dies under mysterious circumstances and is reborn as a baby named Arthur Leywin in the magical continent of Dicathen. Although he begins his second life as an infant in a loving family of retired adventurers, he retains all the memories, wisdom, and emotional scars from his previous life as a cold, isolated, and ruthless ruler. As Arthur grows, he discovers that this new world is filled with mana-based magic, mythical creatures, and political intrigue. Using the knowledge from his past life, he quickly becomes a child prodigy, learning to harness magic at an unprecedented age and impressing powerful figures, including the elven leader Virion Eralith and the director of the prestigious Xyrus Academy, Cynthia Goodsky. However, a peaceful childhood is shattered when a family trip is attacked by bandits, leading Arthur to encounter a mysterious dragon named Sylvia, who sets him on a path to uncover a great hidden danger. The narrative follows Arthur's journey through childhood and adolescence as he enrolls in the Royal Academy, forms bonds with friends and rivals like Tessia Eralith and Elijah Knight, and learns that he is not the only one who has reincarnated in this world.

The setting spans several distinct locations, primarily the human kingdom of Sapin and the elven kingdom of Elenoir within the continent of Dicathen, which is governed by a Council of the major races. The story draws a contrast between Arthur’s peaceful new life with his parents, Reynolds and Alice, and the violent political landscape he slowly uncovers. Notable narrative arcs in the first season include Arthur’s early years mastering magic under the tutelage of his father and Virion, the events of the slave trader incident where he first meets Tessia, his entrance into and time at Xyrus Academy where he confronts bullies and corrupt professors, and the attack on the Academy which forces him to reveal the true extent of his power. The anime also explores the mystery of Arthur's past world, which is revealed not to be a version of our modern Earth but a separate, technologically advanced sci-fi realm, adding a layer of mystery to his original death. As the season progresses, Arthur is recognized as a Lance, a powerful champion of Dicathen, as the kingdom prepares for a looming war against the hostile continent of Alacrya and its mysterious sovereign, Agrona Vritra.