Takumi, a fourth-grade boy, finds the transformation belt he has long admired at a flea market. After rough classmates take it from him, his frustration sparks a transformation pose, and he really does become Thunder Mask. With no idea how to change back, he sets out on an adventure that embraces what he loves.

The moment he gains the object of his admiration, Takumi's everyday life begins to change through his encounter with Thunder Mask.