"Osomatsu-kun" is a comedic anime series centered around the Matsuno sextuplets: Osomatsu, Karamatsu, Choromatsu, Ichimatsu, Jyushimatsu, and Todomatsu. The six brothers, each with distinct personalities, navigate their daily lives in a small town, often engaging in absurd and humorous situations. Osomatsu, the eldest, is the unofficial leader, known for his laziness and mischievous nature. Karamatsu, the second eldest, is self-absorbed and often ridiculed for his attempts to appear cool. Choromatsu, the third, is the most responsible and pragmatic, frequently exasperated by his brothers' antics. Ichimatsu, the fourth, is introverted and cynical, often seen as the "dark" one. Jyushimatsu, the fifth, is eccentric and hyperactive, with a penchant for baseball. Todomatsu, the youngest, is cunning and manipulative, often using his charm to get his way.

The series lacks a continuous overarching plot, instead presenting episodic scenarios that highlight the brothers' interactions with each other and the people around them. Their misadventures range from attempting to find jobs or girlfriends to dealing with supernatural occurrences and rivalries with other characters, such as Iyami, a recurring antagonist with a distinctive "sheeh" catchphrase, and Chibita, a short-tempered friend who runs a local oden cart. The humor often stems from the brothers' exaggerated personalities, slapstick comedy, and satirical takes on societal norms.

The setting is a modest, working-class neighborhood, reflecting the brothers' lower-middle-class status. Their parents, Matsuyo and Matsuzo, are frequently exasperated by their sons' behavior but remain supportive in their own way. The series also features a variety of side characters, including Totoko, a popular girl who often becomes the object of the brothers' affections, and Dayon, a mysterious and powerful figure who occasionally aids or hinders the sextuplets.

"Osomatsu-kun" blends surreal humor with relatable family dynamics, creating a chaotic yet endearing portrayal of sibling relationships. The episodic nature allows for a wide range of comedic scenarios, often breaking the fourth wall or parodying other media. The series' charm lies in its ability to balance absurdity with moments of genuine connection among the brothers, making it a staple of classic anime comedy.