"Girlish Number" centers on Chitose Karasuma, a young and ambitious voice actress who dreams of achieving fame and success in the competitive anime industry. Despite her confidence and self-assured demeanor, Chitose lacks significant experience and often underestimates the challenges of the profession. She secures a leading role in a new anime series, but her initial excitement is tempered by the realization that the production is plagued by poor planning, low budgets, and a lack of creative direction. The series she is working on, titled "Millennium Princess Aria," is a generic fantasy anime with a weak script and uninspired characters, further complicating her aspirations.

Chitose is surrounded by a cast of fellow voice actors, each with their own struggles and perspectives on the industry. Yae Kugayama, her more experienced and pragmatic colleague, serves as a foil to Chitose’s naivety, often providing blunt but necessary advice. Koto Katakura, a cheerful and hardworking newcomer, contrasts with Chitose’s laid-back attitude, while Momoka Sonou, a former child star, grapples with the pressure of maintaining her career as she transitions into more mature roles. The group is managed by Gojou Karasuma, Chitose’s older brother, who works as a talent agent and often finds himself mediating between his sister’s ambitions and the harsh realities of the industry.

As the production of "Millennium Princess Aria" progresses, Chitose and her colleagues face numerous setbacks, including last-minute script changes, uncooperative staff, and the growing realization that the anime is unlikely to succeed critically or commercially. The series explores the behind-the-scenes dynamics of the anime industry, highlighting the disconnect between creative aspirations and the commercial demands of production. Chitose’s journey is marked by moments of self-reflection as she begins to understand the complexities of her profession and the importance of perseverance and humility.

The narrative balances humor and drama, offering a candid portrayal of the challenges faced by voice actors and the often-unpredictable nature of the entertainment industry. Through its characters and their interactions, "Girlish Number" provides insight into the personal and professional struggles of those working in anime, while maintaining a focus on Chitose’s growth as she navigates her career and learns to balance her ambitions with the realities of her chosen path.