In Hinamizawa, in the early summer of 1983, the Hinamizawa school hosts a water gun battle for P.E. class. Keiichi Maebara and Rena Ryuuguu have been struggling to win any club activities, often suffering humiliating defeats. After a brief pep talk from Keiichi, Rena becomes determined to win the water gun battle. When the game starts, Keiichi immediately blasts several children, eliminating them from the game. As he approaches the school yard, Keiichi discovers that Mion Sonozaki has eliminated several contestants herself, including Rika Furude, using a water machine gun. To counter his range disadvantage, Keiichi uncaps his water pistol and throws it at Mion. Water spills from the open cap, splashing on Mion, thus eliminating her. A soaked Satoko Houjou approaches before falling to her knees. Rena stands behind her, armed with a water pistol. Keiichi and Rena engage in a fast-paced fight, though both have difficulty landing a shot on the other. Eventually, the two are both down to their last shot's worth of water. Before the two can use their last shot, however, the bell rings, ending class for the day. Mion declares that the match will be postponed until a later date.
Keiichi notices a scrape on Rena's elbow and insists on giving it proper medical attention. Rika, anticipating such an event, has a first-aid kit. After disinfecting and bandaging the wound, Keiichi and Rena turn to the others and inform them of their punishment for losing the water gun battle. The next day, Keiichi and Rena enter Angel Mort and are greeted by Satoko, Mion, and Rika, all wearing different fetish outfits. The group is approached by Mion's twin sister, Shion Sonozaki. Shion remarks on how close the group has become.
After spending the day with the rest of the club members, Keiichi and Rena head home together. On the way, Keiichi learns that Rena is also a transfer student, having moved to Hinamizawa one year earlier. Keiichi tells Rena about how happy moving to Hinamizawa has made him and asks if she feels the same way. Rena turns to him and smiles, saying that she is indeed happy. Keiichi thinks that, for a brief second, Rena's smile looked sad.
When Rena gets home, she greets her father and shows him a shortcake she bought for him from Okinomiya. While Rena is thinking about how she wants to share her happiness with her father, Rina Mamiya pops out of an adjacent room wearing only a shirt. Rena is visibly disturbed by Rina's presence in the house but insists it doesn't interfere with her happiness. Rina is revealed to be Rena's father's lover. While Rena is happy that her father has found someone since his divorce a few years earlier, she has begun to feel like she no longer belongs in the Ryuuguu household. Rina notices the bandage on Rena's arm and asks why Rena was scratching it so much. Rena was unaware that she was scratching it at all. Rena remarks that, while Rina isn't someone she gets along with, she is grateful to Rina for making her father happy again. However, Rina's increased presence at the house has given Rena the feeling that the house is being invaded.
That night Rena, leaves the house to look for treasure at the garbage dump. While there, Rena's scrape becomes extremely itchy. Rena begins to scratch at her scrape until her fingers are bloody. Eventually, something flies out of the wound and lands on the ground. To Rena's horror, a group of bloody maggots wriggle on the ground before her. Rena screams that there can't possibly be any maggots in her body, as the maggots were all erased by Oyashiro. Rena reveals that she had already lived in Hinamizawa for seven years before moving away to Ibaraki. She believed that her happy life would continue when her family moved to Ibaraki, not knowing that moving to Ibaraki would eventually destroy her family.
- The Japanese title for this chapter, Shiawase na Rena, is spelled without any kanji. Notably, Shiawase is spelled using katakana. This contrasts with the final chapter of Tsumihoroboshi-hen, also entitled Shiawase na Rena. The final chapter's title is instead spelled using the kanji for Shiawase.