In role-playing games, slimes are usually the easiest monster to kill, and because of that, they yield few experience points. But what would happen if you live long enough to keep defeating them for 300 years? After many years of being a corporate slave, Azusa Aizawa abruptly passes away due to severe exhaustion. Seemingly headed for the afterlife, she meets a goddess who bestows her with immortality alongside a peaceful life in another world. There, Azusa enjoys her days tending to her farm, protecting the nearby village, and killing about 25 slimes per day—a routine that continues for at least three centuries. However, this rather monotonous cycle begins to change when Azusa suddenly finds out that she has reached level 99—the maximum possible level—from slimes alone. Despite desperately trying to hide this fact in fear of ending her slow life, rumors of her strength spread nevertheless. Soon enough, various people throughout the continent, like the dragon Laika and the elf Halkara, start to appear at her doorstep—some seeking a battle, others asking for her assistance. Meeting friends and acquaintances who soon become family, Azusa finds she can live a life far better with others than when she was alone. Suddenly dying from overwork, salarywoman Azusa Aizawa finds herself before an angel, who allows her to reincarnate into a new world as an immortal witch, where she spends her days killing slimes for money on an otherwise eternal vacation. But even the minimal experience points from slimes will add up after hundreds of years, and Azusa discovers that she accidentally reached the maximum level! Fearing that her strong abilities will attract work and force her back to a life of overexertion, she decides to hide her strength in order to preserve her peaceful lifestyle.