Having defeated countless enemies, obtaining wealth, fame, and everything else imaginable, the most powerful king, Grey, abruptly dies one day. He then awakens suddenly, having reincarnated in a world where magic exists. He is given the name Arthur, and begins living a second life, with loving parents.

The Leywin family travels to the floating city of Xyrus so that Arthur can begin his formal education in magic. The adventuring party that Reynolds and Alice belong to, the Twin Horns, accompany them as guards, when they run into something unexpected...

During the battle with the bandits, Arthur falls off of a cliff. When he awakens he encounters a mysterious beast, clad in armor. Her name is Sylvia. She helps Arthur, and seems to know where his family is, but who is she really...?

Having parted ways with Sylvia, Arthur hears a girl's scream in the woods. Upon rushing to the voice, he encounters an elf girl named Tessia, who has been captured by slavers. With Sylvia's words to "seek light in his new life" ringing in his ears, Arthur decides to challenge them by himself.

Arthur arrives in the Elenoir Kingdom with Tessia, before being arrested by elvish soldiers who harbor deep resentment toward humans. Things are looking tense until Elenoir's former ruler, Virion, has him released. Arthur is then invited to the kingdom's castle. But the pain in his mana core seems to be getting worse...

After being informed of a way to let Reynolds and Alice know about his survival, Arthur joins Virion and they head to a witch's house. Using special sorcery, he is able to see his parents. Witnessing the depth of their grief, Arthur decides that he will train under Virion to protect those he cares about, and to control the dangerous power growing within him.

It's been three years after Arthur began his training in the Kingdom of Elenoir, having worked alongside Tessia as they both grew stronger. Arthur enlists Virion's help in order to awaken the beast that slumbers within his mana core. Then, one night, the stone Sylvia gave him begins to shine…