"Hello Hari-nezumi" is a manga series that centers around the life of a young girl named Haru, who discovers she has the ability to transform into a hedgehog. This transformation occurs whenever she feels intense emotions, particularly stress or embarrassment, which complicates her daily life. Haru struggles to balance her ordinary school life with her newfound ability, often finding herself in humorous and awkward situations as she tries to keep her secret hidden from her classmates and family.

The narrative explores Haru's relationships with those around her, including her best friend, Yuki, who remains unaware of Haru's condition but provides unwavering support. Another key character is Kaito, a classmate who becomes suspicious of Haru's strange behavior and begins to investigate, adding tension to the story. As Haru navigates her dual life, she encounters other individuals with similar transformative abilities, revealing a hidden world of people who can shift into animals. This discovery leads Haru to question the origins of her power and its connection to a larger, mysterious phenomenon.

The plot delves into themes of self-acceptance, friendship, and the challenges of adolescence, as Haru learns to embrace her unique identity while striving to maintain a sense of normalcy. The series balances lighthearted moments with deeper emotional undertones, creating a narrative that is both engaging and relatable. Through her journey, Haru grows more confident in herself, ultimately using her abilities to help others and confront the mysteries surrounding her transformation.