"Soul Eater Not!" is a spin-off of the original "Soul Eater" series, set in the same universe but focusing on a different group of characters and a more slice-of-life narrative. The story takes place at the Death Weapon Meister Academy (DWMA), a school where students train to become either weapon meisters or demon weapons. The series centers on Tsugumi Harudori, a shy and inexperienced girl who arrives at the academy as a "NOT" student, a designation for those still learning to control their abilities and not yet ready for combat missions.

Tsugumi is a demon weapon capable of transforming into a guillotine, but she struggles with her powers and self-confidence. She is assigned to a shared dormitory with two other NOT students: Meme Tatane, a cheerful and energetic meister, and Anya Hepburn, a reserved and aristocratic weapon who transforms into a halberd. The three form a team and navigate the challenges of school life, training, and forming bonds with one another. Their interactions highlight themes of friendship, self-discovery, and personal growth.

The plot unfolds through episodic adventures, often involving lighthearted missions and school activities, contrasting with the darker, action-heavy tone of the main "Soul Eater" series. The trio encounters familiar characters from the original series, such as Maka Albarn and Soul Eater, but the focus remains on Tsugumi, Meme, and Anya as they develop their skills and relationships. The series also introduces new characters, including Clay Sizemore, a meister who becomes a close ally to the group, and Shaula Gorgon, a teacher with a mysterious past.

While the stakes are lower compared to the original series, "Soul Eater Not!" explores the daily lives of students at the DWMA, offering a more intimate look at the academy's structure and culture. The narrative occasionally touches on the broader conflicts of the "Soul Eater" universe, such as the presence of witches and kishin, but these elements serve as background rather than central plot points. The series concludes with the trio gaining confidence in their abilities and solidifying their friendship, leaving their future adventures open-ended.