"My One-Hit Kill Sister" centers on Asahi Ikusaba, an ordinary high school student who finds himself transported to a fantasy world alongside his older sister, Maya. In this new realm, Maya possesses an overwhelming power that allows her to defeat any enemy with a single strike, earning her the title of "One-Hit Kill Sister." Despite her immense strength, Maya remains fiercely protective of Asahi, often prioritizing his safety over her own ambitions. The siblings navigate a world filled with magic, monsters, and adventurers, where Asahi struggles to find his place while relying on Maya's unparalleled abilities.

The narrative explores the dynamics between the siblings as they encounter various challenges and allies in their journey. Maya's overpowering strength often leads to comedic and lighthearted moments, as her abilities trivialize most threats. However, the story also delves into the emotional bond between the siblings, highlighting Maya's unwavering devotion to Asahi and his growing determination to stand on his own. The fantasy world introduces a mix of traditional RPG elements, including guilds, quests, and magical creatures, providing a backdrop for the siblings' adventures.

Key characters include Maya Ikusaba, whose cheerful and carefree demeanor contrasts with her devastating combat prowess, and Asahi Ikusaba, who serves as the more grounded and relatable protagonist. Supporting characters, such as fellow adventurers and guild members, add depth to the world and occasionally challenge the siblings' perspectives. The series balances action, humor, and heartfelt moments, creating a narrative that appeals to fans of both fantasy and slice-of-life genres. The overarching plot focuses on the siblings' journey to adapt to their new world while strengthening their bond and uncovering the mysteries surrounding their sudden arrival.