"JimiHen—!! ~Jimiko o Kaechaū Jun Isei Kōyū~" centers on the life of Jun, a high school student who struggles with his self-image and social standing. Despite his efforts to fit in, Jun is often perceived as dull and unremarkable, earning him the nickname "Jimiko" (plain girl) among his peers. His life takes an unexpected turn when he encounters a mysterious app that allows him to transform into a strikingly attractive version of himself. This newfound ability grants Jun access to a world of popularity, attention, and social validation that he had previously only dreamed of.

As Jun navigates this dual identity, he begins to experience the complexities and consequences of living two vastly different lives. In his transformed state, he attracts admiration and romantic interest, but he also faces challenges in maintaining his relationships and reconciling his true self with the idealized persona he presents. The story explores themes of identity, self-worth, and the pressures of societal expectations, as Jun grapples with the question of whether his newfound popularity is worth the emotional toll it takes on him.

The narrative delves into Jun's interactions with various characters, including classmates, love interests, and rivals, each of whom plays a role in shaping his journey. His relationships are tested as he struggles to balance his authentic self with the facade he creates through the app. The series also examines the impact of appearance on social dynamics, highlighting the superficiality and judgment that often accompany high school life.

Through its exploration of Jun's transformation and the ensuing conflicts, "JimiHen—!!" offers a nuanced portrayal of the challenges of self-acceptance and the pursuit of validation in a world that often prioritizes outward appearances over inner qualities. The story unfolds with a mix of humor, drama, and introspection, providing a compelling look at the complexities of adolescence and the search for identity.