"Hakkutsu Oppai Daijiten" is a manga series that blends comedy, ecchi, and slice-of-life elements, centering around the protagonist, Kenta, a high school student with an unusual passion for breasts. Kenta’s fascination leads him to create a comprehensive encyclopedia dedicated to documenting and categorizing breasts, which he titles the "Oppai Daijiten." The narrative explores his efforts to gather data for this project, often placing him in humorous and awkward situations as he interacts with female classmates and other characters.

The story unfolds in a school setting, where Kenta’s eccentric project becomes a focal point for various comedic encounters. His interactions with female characters, including classmates and teachers, are driven by his quest to understand and document the diversity of breasts. These interactions are often lighthearted, with misunderstandings and playful banter forming the core of the humor. Despite the ecchi premise, the series maintains a comedic tone, avoiding overtly serious or dramatic themes.

Supporting characters include Kenta’s friends and classmates, who react to his project with a mix of curiosity, embarrassment, and occasional exasperation. The female characters, while initially skeptical or wary of Kenta’s intentions, often find themselves drawn into his project, contributing to the comedic dynamic. The series does not delve deeply into character backstories or complex relationships, instead focusing on episodic scenarios that highlight Kenta’s quirky obsession and the resulting comedic situations.

The plot progresses through a series of vignettes, each centered around Kenta’s attempts to expand his encyclopedia. These episodes often involve misunderstandings, slapstick humor, and playful innuendo, with the narrative maintaining a light, comedic tone throughout. The series does not aim for a overarching plot or dramatic resolution, instead relying on its episodic structure to deliver consistent humor and entertainment.

Overall, "Hakkutsu Oppai Daijiten" is a comedic manga that uses its ecchi premise as a vehicle for humor, focusing on the protagonist’s eccentric project and the resulting interactions with the characters around him. The series is characterized by its lighthearted tone, episodic structure, and emphasis on comedic situations rather than deep narrative or character development.