Newly enrolled in junior high, Haru Nitta is unsettled by the changes around him. One evening, while sitting alone on a park bench, a mysterious young woman with a languid, downbeat air calls out to him and thrusts a Beyblade into his hands. Her surprising passion for toys and games quickly reveals her true identity: a twenty-seven-year-old avid reader of CoroCoro Comic who treasures children's hobbies above all else. She adopts the nickname "older sister" and insists on calling Haru "boy," dragging him into a series of playful encounters centered on classic and contemporary CoroCoro-linked pastimes.

The story unfolds in everyday suburban settings—parks, hobby shops, and local event spaces—where the older sister introduces Haru (and the reader) to a rotating lineup of toys and games. These range from Beyblade, yo-yos, and Mini 4WD to trading card games like Duel Masters and Cardfight!! Vanguard, as well as cultural touchstones such as Yo-kai Watch and references to iconic CoroCoro manga series. Each chapter or story arc typically revolves around a specific hobby or a CoroCoro-themed parody, with the older sister's encyclopedic knowledge and infectious enthusiasm driving the comedy. Haru, initially shy and inexperienced, finds himself both bewildered and gradually drawn into the joy of play.

The main characters are Haru Nitta, a conscientious boy raised by strict parents who never allowed him to indulge in such toys, and the unnamed "downer older sister," whose droopy-eyed, deadpan demeanor contrasts sharply with her hyper-focused, almost obsessive love for games. She is often seen wearing a white shirt, tight miniskirt, and black tights, and she carries Beyblades inside her jacket. Her baffling mix of seductive poses and totally oblivious, hobby-driven behavior creates much of the humor. Supporting characters include other children and adults who occasionally join the play sessions, but the core dynamic remains Haru and the older sister.

Notable narrative arcs include the older sister's attempts to teach Haru the finer points of competitive Beyblade, a foray into building and racing Mini 4WD cars, an elaborate recreation of a corny but heartfelt scene from a CoroCoro gag manga, and a visit to the real-life CoroCoro BASE shop. Throughout these episodes, the manga balances slapstick comedy with genuine nostalgia, celebrating the culture of children's toys without cynicism. At the same time, Haru's growth from a withdrawn adolescent to someone who values friendship and fun is gently charted, while the older sister remains a static but delightful catalyst. The story is ongoing, with each new chapter introducing another hobby or CoroCoro franchise to explore.