"Katteni Kaizō" is a surreal and comedic anime series that revolves around the eccentric and unpredictable life of Kaizō Ningen, a self-proclaimed "modified human" with the ability to alter his body at will. The narrative unfolds in a series of loosely connected vignettes, each highlighting Kaizō's bizarre adventures and interactions with those around him. His modifications range from the absurd, such as elongating his limbs or transforming into inanimate objects, to the grotesque, often leading to chaotic and humorous outcomes.

Kaizō's primary companion is his younger sister, Aiko, who frequently finds herself exasperated by his antics yet remains a grounding presence in his life. Their relationship is a mix of sibling rivalry and genuine care, with Aiko often serving as the voice of reason amidst Kaizō's relentless pursuit of absurdity. Other recurring characters include Kaizō's classmates and acquaintances, who are drawn into his schemes and transformations, often to their dismay.

The plot lacks a conventional overarching structure, instead focusing on episodic scenarios that emphasize Kaizō's eccentricity and the absurdity of his world. Themes of identity, individuality, and the boundaries of human capability are explored through Kaizō's constant self-modification, though these are presented in a lighthearted and often nonsensical manner. The series thrives on its unpredictable humor, blending slapstick comedy with surreal visuals and situations.

Despite its comedic tone, "Katteni Kaizō" occasionally delves into moments of introspection, particularly in episodes where Kaizō's modifications lead to unintended consequences or emotional revelations. These moments, though fleeting, add depth to an otherwise chaotic narrative. The series maintains a fast-paced and irreverent tone, appealing to viewers who enjoy unconventional storytelling and absurd humor.