In a peaceful fantasy world, Azusa Aizawa, a former overworked office worker in Japan, dies from exhaustion and is reincarnated into a new life as an immortal witch. Seeking a stress-free existence, she settles in the countryside, adopting a simple routine of killing low-level slimes daily to earn a modest living. Over time, this repetitive task inadvertently leads her to accumulate an immense amount of experience, eventually maxing out her level to an extraordinary degree. Despite her overwhelming power, Azusa remains committed to her quiet, unassuming lifestyle.

Her peaceful routine is disrupted when her reputation as the "Witch of the Highlands" attracts attention. A dragon named Laika challenges her to a duel, only to be defeated and subsequently become Azusa’s loyal apprentice. This encounter marks the beginning of Azusa’s growing circle of companions. She adopts two slime spirits, Shalsha and Falfa, who evolve into humanoid forms and become her daughters. Other characters, such as the elf Halkara, the demon girl Beelzebub, and the ghost Flatorte, join her household, each bringing their own quirks and challenges.

The series explores the daily lives of Azusa and her eclectic group of friends and family, blending slice-of-life moments with lighthearted fantasy adventures. While Azusa initially resists the disruptions to her tranquil life, she gradually embraces the bonds she forms with those around her. The narrative emphasizes themes of community, found family, and the joys of a balanced life, contrasting Azusa’s past of overwork with her newfound contentment. Despite occasional conflicts or misunderstandings, the tone remains warm and comedic, with Azusa’s overwhelming power often serving as a humorous backdrop to her desire for simplicity.