Naruse Akari is a junior high school student living in Otsu City, Shiga Prefecture, near Lake Biwa. Academically gifted and athletically talented, she is also fiercely eccentric and follows her own logic without concern for what others think. At the start of summer vacation, she makes a strange declaration to her childhood friend and neighbor, Shimazaki Miyuki: I am going to dedicate this summer to Seibu. The Seibu in question is the local Seibu Department Store, scheduled to close permanently on August 31. Aiming to show her appreciation for the landmark, Naruse resolves to appear on a local television program broadcasting live from in front of the store every single day until it shuts, donning a Seibu Lions baseball uniform to ensure she is featured.

This summer project is only the beginning. Naruse, who once appeared on TV as a champion soap-bubble blower and whose graduation yearbook listed her ambition as to live to be 200, continues to drag the reluctant Shimazaki into a series of all-in schemes. Her next target is comedy. The pair form a manzai duo named Zezekara, short for Zeze kara kimashita or We come from Zeze, after the unusual name of the train station near their apartment building, and enter the qualifying rounds of a major comedy competition. Naruse pursues every endeavor with complete seriousness, whether it is acing her end-of-term exams nearly perfectly or shaving her head for an experiment. Despite being regarded as an oddball by classmates, her magnetism, energy, and open-eyed optimism gradually win those around her over.

The narrative follows Naruse from her second year of junior high school through her entrance into the selective Prefectural Zeze High School. Key episodes include her single-minded campaign to be featured on the daily farewell broadcasts to the closing Seibu store, her and Shimazakis attempt to compete in the M-1 Grand Prix as a comedy duo, and her adjustment to a new school where her unconventional ways initially draw skeptical looks but eventually draw her peers into her eccentric little plots that sweep across small-town life. The sequel arc sees Naruse working as a supermarket cashier while studying for college entrance exams, aiming for Kyoto University so she can continue living at home. She is unexpectedly selected as a Lake Biwa Tourism Ambassador, a position that leads to a rivalry-cum-friendship with a privileged girl named Shinohara Karen and a preliminary competition representing the Kansai Region. Late in the year, Naruse suddenly disappears, leaving only a note telling her friends not to look for her, prompting Shimazaki and the entire community to band together in a search that culminates in a vivid and heartwarming reunion.