"Shampoo Ōji" centers around the life of Kei Shindō, a high school student who works part-time at a hair salon. Kei's life takes an unexpected turn when he encounters a mysterious talking cat named Lion, who claims to be a prince from the Cat Kingdom. Lion reveals that he has been cursed and transformed into a cat, and he needs Kei's help to break the spell. The curse can only be lifted if Lion performs acts of kindness and gains the gratitude of humans, which he struggles to achieve due to his arrogant and self-centered nature.

As the story progresses, Kei becomes increasingly involved in Lion's quest, often finding himself in bizarre and comedic situations. Lion's attempts to help others are frequently misguided, leading to misunderstandings and chaos. Despite his flaws, Lion gradually begins to develop a sense of empathy and responsibility, influenced by Kei's kindness and patience. The dynamic between the two characters forms the core of the narrative, blending humor with moments of genuine emotional growth.

The series also introduces a supporting cast of characters, including Kei's classmates and coworkers, who add depth to the story. Each episode typically features a self-contained storyline, often involving Lion's efforts to assist someone in need, albeit in his own unconventional way. These episodic adventures allow for a variety of comedic and heartfelt moments, showcasing the evolving relationship between Kei and Lion.

Throughout the series, themes of personal growth, friendship, and the importance of understanding others are explored. The interactions between Kei and Lion highlight the challenges of communication and the value of compassion. While the tone is predominantly lighthearted, the narrative occasionally delves into more serious topics, providing a balanced mix of comedy and drama.

"Shampoo Ōji" is characterized by its unique premise and the endearing, albeit flawed, protagonist in Lion. The series effectively combines humor with meaningful character development, making it an engaging watch for audiences interested in stories that blend the fantastical with everyday life.