Azusa Aizawa, a former overworked office worker in Japan, dies from exhaustion and is reincarnated into a fantasy world as an immortal witch. Seeking a peaceful life, she settles in the remote Highlands of Flatta, where she spends her days killing low-level slimes for a modest income. Over the course of 300 years, her consistent slime hunting inadvertently causes her to max out her level, making her one of the most powerful beings in the world. Despite her immense strength, Azusa remains committed to her simple, tranquil lifestyle.

Her peaceful existence begins to change when her reputation as the "Witch of the Highlands" attracts attention. A dragon named Laika challenges her to a duel, but after being defeated, Laika becomes Azusa’s first friend and disciple. Soon after, Azusa adopts two young slime spirits, Falfa and Shalsha, who were born from the countless slimes she had slain. The family grows further when a demon girl named Halkara, who runs a potion business, moves in after being saved by Azusa. Later, a ghost named Flatorte, a rival dragon to Laika, joins the household after being defeated and humbled.

As Azusa’s household expands, she becomes increasingly involved in the lives of those around her. She helps Halkara resolve business troubles, supports Falfa and Shalsha in their growth, and mediates conflicts between Laika and Flatorte. Despite her reluctance to engage in grand adventures, Azusa’s kindness and overwhelming power often lead her to assist others, whether it’s resolving misunderstandings with neighboring towns, aiding a struggling elf, or even confronting a demon lord. Throughout these events, Azusa maintains her desire for a quiet life, though her growing circle of friends and family ensures that her days are anything but uneventful.

The series balances slice-of-life elements with light fantasy adventures, emphasizing themes of friendship, family, and the pursuit of happiness. Azusa’s journey is less about epic battles or world-saving quests and more about finding joy in the small, everyday moments with her unconventional family. The narrative’s charm lies in its gentle humor, heartwarming interactions, and the contrast between Azusa’s overwhelming power and her desire for a simple, fulfilling life.