"Penguin Girl" centers around the life of a high school student named Eiji Yamashita, who becomes entangled in an unusual situation when he encounters a mysterious girl named Aoi Yukino. Aoi is known as the "Penguin Girl" due to her distinctive penguin-shaped backpack and her quirky, penguin-like mannerisms. Despite her eccentric appearance, Aoi is a popular and confident figure at school, often drawing attention with her bold and carefree attitude.

Eiji, a reserved and somewhat awkward teenager, finds himself drawn to Aoi’s unconventional personality. Their relationship begins when Eiji accidentally discovers Aoi’s secret: she works as a gravure model, posing for photoshoots in revealing outfits. This revelation creates a dynamic where Eiji becomes both a confidant and a source of support for Aoi, as she navigates the challenges of balancing her public persona with her private life. The series explores themes of identity, self-expression, and the pressures of societal expectations, particularly through Aoi’s dual life as a model and a high school student.

As the story progresses, Eiji and Aoi’s bond deepens, with Eiji often finding himself in comedic and awkward situations due to Aoi’s unpredictable behavior. The narrative also introduces other characters, such as Eiji’s childhood friend, Ruriko Yamashita, who harbors feelings for him, and Aoi’s rival, Natsuki Ayanokouji, who adds tension to the story. The interactions between these characters create a blend of humor, drama, and lighthearted moments, as the series delves into the complexities of teenage relationships and personal growth.

The plot is driven by the evolving dynamics between Eiji and Aoi, as well as the challenges they face individually and together. Aoi’s struggle to maintain her dual identity and Eiji’s efforts to understand and support her form the emotional core of the series. Through its mix of comedy, romance, and slice-of-life elements, "Penguin Girl" presents a story that balances lighthearted entertainment with moments of introspection and character development.