"Arakawa Under the Bridge" centers on Kou Ichinomiya, a wealthy and prideful young man raised with the belief that he must never be indebted to anyone. This principle is tested when he is rescued from a near-drowning incident by Nino, a mysterious girl living under the Arakawa Bridge in Tokyo. Feeling obligated to repay her, Kou moves under the bridge to live alongside Nino and the eccentric community of outcasts who reside there. The series explores Kou’s attempts to adapt to this unconventional lifestyle while navigating the quirks and dynamics of the bridge’s inhabitants.

The bridge community is populated by a colorful cast of characters, each with their own peculiarities. Nino, Kou’s rescuer, claims to be from Venus and exhibits a detached, otherworldly demeanor. The self-proclaimed village chief, known simply as "Chief," wears a kappa costume and insists he is a real kappa. Other residents include Sister, a guitar-wielding nun with a violent streak; Star, a former soldier obsessed with military tactics; and Maria, a stoic woman who runs a makeshift bar. Each character contributes to the surreal and comedic atmosphere of the series.

As Kou integrates into the community, his interactions with Nino and the others challenge his rigid worldview and force him to confront his own vulnerabilities. The narrative unfolds through a series of episodic adventures, blending absurd humor with moments of introspection. Kou’s growing affection for Nino becomes a central thread, as he grapples with her enigmatic nature and his own feelings of inadequacy. The series balances its comedic tone with subtle explorations of themes such as identity, belonging, and the meaning of family.

The plot is driven by the juxtaposition of Kou’s privileged background with the chaotic, free-spirited lifestyle of the bridge dwellers. His attempts to impose order and logic on the community often lead to comedic misunderstandings, while the residents’ unconventional perspectives gradually influence his outlook. The series maintains a lighthearted tone, punctuated by surreal gags and visual humor, but also delves into the emotional complexities of its characters. Through its eccentric cast and offbeat storytelling, "Arakawa Under the Bridge" presents a unique blend of comedy and heartfelt moments, anchored by Kou’s journey of self-discovery and his evolving relationships with the bridge’s inhabitants.