"Marimo no Hana: Saikyō Butō-ha Shōgakusei Densetsu" centers on Marimo, a young elementary school student who possesses extraordinary strength and a strong sense of justice. The narrative unfolds in a school setting where Marimo, despite her small stature, becomes a protector of her classmates, often standing up to bullies and troublemakers. Her immense physical power, which she struggles to control, sets her apart from her peers and leads to both comedic and dramatic situations.

The story explores Marimo's daily life as she navigates the challenges of being a child with unique abilities. Her strength often causes unintended consequences, such as accidentally breaking objects or overwhelming her opponents, which adds a layer of humor to the series. Despite her power, Marimo remains grounded and kind-hearted, striving to use her abilities for good. Her interactions with classmates, teachers, and even rival students highlight her determination to maintain fairness and protect those around her.

Key characters include Marimo's classmates, who often rely on her for help, and her teacher, who tries to guide her in balancing her strength with responsibility. The series also introduces rival students who challenge Marimo, leading to confrontations that test her resolve and abilities. These encounters often serve as opportunities for Marimo to grow and learn more about herself and the impact of her actions.

The plot is episodic, with each episode presenting a new challenge or conflict for Marimo to overcome. Themes of friendship, perseverance, and self-discovery are woven throughout the narrative, emphasizing the importance of using one's strengths for the benefit of others. The series balances lighthearted moments with more serious undertones, creating a dynamic and engaging story that appeals to a wide audience.