Gyo Ichinomiya, a sergeant of the Ichinomiya Group, a large zaibatsu, has lived his whole life under the strict instruction of his father, “Never owe a debt to another person. One day, a girl named Nino, who claims to be a Venusian, saves his life when he meets her on a bridge on the Arakawa River. Unintentionally answering “yes,” Gyoko retreats under the Arakawa bridge, where strange people are living.

Gyoko was given the name “Riku” by the village chief, a man in a kappa costume who was in charge of the Arakawa riverbed, and welcomed him as a member of the group. To his surprise, the squatters who had gathered at the riverbed were all somewhat strange people.

Riku is bewildered, but gradually adjusts to life on the riverbed. As he teaches Tetsuo, Tetsuro, Stella, and Piko how to fly a rocket out of a plastic bottle, he is reminded of his own childhood. Riku's feelings gradually change as he has an experience in which his thoughts suddenly match Nino's and hears about Piko's love for the village chief.

Riku has become like a teacher for Tetsuo, Tetsuro, Stella, and the other children. While on a “treasure hunt” as a social tour as it happens, he encounters the unexpected true face of Shiro, the man who always draws a white line in the lime. In Riku's classroom, Stella writes an essay imagining how Billy and Jacqueline came to Arakawa.

Golden Week, though the cherry blossoms are in bloom. It had been a month since Riku came to live at the riverbed. Riku is told by the sisters, “In the first place, people are full of debts when they are born,” which is the exact opposite of his own value system. It is true that everyone here is living together physically and mentally.

Riku suddenly bursts into tears as Nino washes his hair. Nino, who has become sentimental, asks him out on a date. However, Venusians do not have the concept of a date, and they cannot leave the Arakawa River due to its gravitational pull and magnetic field.

Since living on the banks of the Arakawa River, Riku “has been having more and more memories that he would like to ‘remember’ later. Although he and Nino have grown completely closer, Riku is troubled by the fact that the place is owned by the state and the residents are still trespassers. Meanwhile, Piko has a crush on the village chief. With the encouragement of the Last Samurai, P-ko decides to make a move on him, but she is crushed and becomes a bit of a wimp.

The season has changed and it is the rainy season. Residents gathered in Rick's tent. They say that a thing with a raft-like function (like Noah's Ark) is hidden in the place where he has built his tent. They say that this is in preparation for the occasional visitors who will come to wreak havoc on their lives here.

After a hard day of exterminating a nest of ferocious wasps in the riverbed, the sky was full of stars. While everyone was having a great time observing the stars, Maria and Sister kissed each other in a little happening. Stella, who loves Sister, is shocked and cries. As Rikku watched Nino soothing her, the memory of her birth suddenly came back to her. The face of his mother, who had lost her life at the same time he was born, looked exactly like Nino's.

We find out that the rocket taking everyone to Venus has a weight limit! The residents split into teams to try out different dieting techniques. What kind of results can we expect? And what kind of aftereffects might the twins experience!?