In a world where every person is born with exactly eight skill holder slots, inhabitants survive by equipping skill orbs such as Strengthen Arm or Fire Magic into those slots, with the total star value of all equipped skills never exceeding the number of slots available. The ten-year-old protagonist Reiji has spent his entire life as a slave in a mine that produces these skill orbs, bound by contract magic that prevents him from using any skills. Alongside his cherished older sister figure, a fellow slave named Larc, he endures grueling days of forced labor with no hope of change.

A rebellion erupts in the mine when the noble master overseeing it is killed in a sudden cave-in. Amid the chaos, Reiji stumbles upon a ten-star overlimit skill orb called World Ruler, a power that far exceeds the normal eight-slot limit. Upon absorbing it, he not only gains the ability to instantly comprehend any phenomenon and copy the skills of others, but he also recovers the memories of his past life as a seventeen-year-old high school student who died in an accident and was reincarnated into this world. The child slave is, in truth, a reincarnator.

Using his newfound power, Reiji escapes the mine, but in the turmoil he is separated from Larc, the only person he had ever trusted. Driven by the earnest wish to reunite with her and forge a genuine brother-sister bond, he sets out on a journey across distant lands. Along the way, he must conceal the existence of his overlimit ability, as such transcendent power draws the unwanted attention of nations and those who would exploit or destroy him. His adventure becomes a battle not only to find Larc but to navigate a world where the rules of skill holders, the machinations of nobles, and the hidden truths of his own reincarnation all converge.