Ko Ichinomiya's family motto is "Never be indebted to anyone," but after losing his pants and falling into the Arakawa River, he quickly finds himself in debt to his savior, the cutely insane Nino who happens to live under the bridge. To repay her, he vows to help her with her desire to "experience love." Along the way he'll meet the river's other residents, including a hot-blooded kappa, a Sister in drag, and a literal rock star.
Noticing Nino's habit of sleepwalking into his apartment, Ric suggests she move in with him. The guys participate in a marathon run, where Ric gets into a fierce rivalry with Star.
Ric encounters the Amazoness. Captain of the Earth Force shows up under the bridge looking to fight Nino and the Venusians.
Ric finds a cassette player belonging to Nino containing messages for a supposed Venusian. Nino becomes angry at him and climbs on top of a telegraph pole and refuses to come down. As an apology, Ric makes a tape of his embarrassing secrets for Nino to listen to.
P-ko writes a script for a movie and gets Ric to be the director. Later, the gang holds a haunted house which Ric is escorted through, with Star hoping to scare him in front of Nino.
The Captain of the Earth Force starts living in the riverbank after giving the Mayor and the gang manga with them as main characters and revealing that he is a well-known manga author. Later, Billy and Jacqueline get together for their anniversary and Ric finds out they had previously been both members of a yakuza group.
Under Seki's orders, Shimazaki researches about Nino, but she is threatened by a strange man not to go any further. Meanwhile, Nino reportedly receives a tape from her parents and tells Ric she has to leave the riverbank and return to Venus, asking him to come with her. To Ric's surprise, everyone else starts inviting themselves to come along.
The Arakawa residents decide to do a physical checkup to prepare for their trip to Venus, and Ric and Star end up trying to 'outsick' each other to get Nino's attention. Later, the girls spend a week in Sister's basement, though after six days everyone drops out except Nino. When it's the boys turn, Ric tries using P-ko's logbook to strike up a conversation, with embarrassing results.
Amazoness appears at the riverbank and becomes one of Ric's students, revealing she has a crush on him. She has her Tengu henchmen hypnotize Ric to become attracted to her, manipulating him to the point where they get married. Nino competes with Amazoness to prove how much she loves Kou. Despite the chance he would otherwise have, Star decides to stop the Tengu's hypnosis, whilst Ric's true feelings break him out of his hypnosis.
A package arrives for Whitey, and Ric is charged with delivering it. Ric realizes it's from Whitey's wife and assumes it's their divorce papers, but is surprised to find that his wife accepts their long-distance relationship. Later, P-ko prepares a tea party for all the riverbed residents. However, Last Samurai and Mayor head off to have their own tea party, dedicated to wabisabi. Ric and Star go with them as well, and end up talking like they know more about it than they really do.
With an apparent 500kg weight limit for the rocket to Venus, everyone goes on a diet so they can all go together. While Ric and the Mayor lose weight at a spa, the girls suffer through a strawberry only diet while the others have survival training with Sister. As a result of Sister's training, one of the Metal Brothers, Tetsuo, gains powerful muscles, and he soon confronts Stella to see who is more powerful. In order to stop their fighting, his brother Tetsuro builds up his muscles and manages to stop Tetsuo by showing a stronger love for his muscles.
As the Arakawa residents hold a Bon Festival, Star and Amazoness team up to try and split up Ric and Nino. In the end though, Amazoness ends up falling for Star, but can't bring herself to confess her feelings. She sets up a beach shack up stream, where she arranges to be rescued by Star so she can confess her feelings, giving him a good luck charm.
The Mayor decides to hold a tournament to see who is strongest on the riverbank. The winner will be granted one wish, something the rest of the members must help make come true. In the first round, the weaker candidates fight in a battle royale, which P-ko ends up winning. She then faces the stronger candidates in a balloon popping match. Both P-ko and the Mayor pop each others balloons at the same time, and both lose. Two candidates now remain for the title of King, Maria and Sister.
Although Sister puts up a valiant defense against Maria's physical attacks, her insults help her win. Although her royal decree turns out to be to make the residents make clothes for her sheered sheep, she forces Ric, Star, Sister and Last Samurai to be sheepdogs. Afterwards, the residents send their wishes down the river. When Nino's wish for everyone to be together forever gets stuck in the middle of the river, Ric paddles out and frees it for her, reassuring her of that promise.
