"Ningen Kyōki Katsuo!" centers on Katsuo, a young man whose life takes a bizarre turn when he discovers he has the ability to transform into a giant, monstrous version of himself. This transformation occurs whenever he experiences intense emotions, particularly anger or frustration. The story explores Katsuo's struggle to control this power while navigating the chaos it brings to his everyday life. His monstrous form, often referred to as "Katsuo the Monster," becomes both a source of fear and fascination for those around him, leading to a series of unpredictable and often comedic situations.

The narrative delves into Katsuo's relationships with his family, friends, and classmates, who react to his transformations with a mix of awe, fear, and humor. His mother, in particular, plays a significant role, often providing a grounding presence amidst the absurdity. Katsuo's best friend, a loyal and somewhat eccentric individual, frequently finds himself caught up in the fallout of Katsuo's transformations, adding a layer of camaraderie and comic relief to the story.

As the series progresses, Katsuo's transformations begin to attract the attention of various groups, including scientists, government agents, and even rival monsters. These encounters escalate the stakes, forcing Katsuo to confront the implications of his power and the responsibilities that come with it. The story balances its comedic elements with moments of introspection, as Katsuo grapples with questions of identity, self-control, and the nature of humanity.

The manga employs a mix of slapstick humor, surreal scenarios, and occasional action sequences to keep the narrative engaging. Katsuo's transformations are often exaggerated and over-the-top, emphasizing the absurdity of his situation while also highlighting his vulnerability. The series maintains a lighthearted tone overall, even as it explores deeper themes of self-acceptance and the challenges of growing up.

Through its episodic structure, "Ningen Kyōki Katsuo!" presents a series of self-contained yet interconnected stories, each contributing to Katsuo's development as a character. The manga's blend of humor, heart, and occasional chaos makes it a distinctive entry in the genre, offering a unique take on the concept of transformation and its impact on personal and social dynamics.