Because Aihara has a loose baby tooth, she asks Ryōta to stay after school and uses him to help pull it out. During school, in order to shift the spotlight from himself, Ryōta claims that guys care about collarbones and not breasts. After losing the match against Natsumi during recess, Ryōta is forced to accompany her as punishment to run an errand during which they get locked in the P.E. supply closet. Megumi is worried about gaining three kilograms due to an incorrectly balanced scale. Chika tries to trick Ryōta into kissing her but is interrupted.
Ryōta, winning a red superball, has a duel with Kōji to see who can bounce their superball the highest. Later at school, Ryōta, filling his bowl with soup to the brim, accidentally spills milk and soup on Megumi and Yūki. As he heads to going to apologize to them, he instead finds Natsumi who was stripped for a change of clothes by the two. At the Yūki's suggestion, Kazumi who at first is noting down in her diary her observations on the growth of a plant, begins to experiment on Ryōta as he sleeps. Having never seen her smile before, Ryōta, Kōji, Tsukasa, and Chika try to get Kazumi to smile through various means. Ryōta dreams that after school ends, he and Chika begin to walk home together for the first time in a while and end up going to her house. There he imagines that she begins making advances on him. When he wakes up, Chika asks him for his old keychain.
Chika gets Ryōta to let her on his back so that she can reach a book on a high shelf. While warming up in the nurse's office, the girls decide to give each other a surprise physical. Struggling with a cold, Ryōta visits the nurse's office twice only to be repelled by the girls, who are measuring each other twice. Chika tries to trick people into eating chalk but Ryōta is the only who falls for it. After taking it out is anger on Chika's Valentine's Day chocolates, he later earns her forgiveness. After little kids flip their skirts, Yūki and Chika ask their friends for their opinions on what age to stop forgiving them. When asking the boys, they refuse to believe Ryōta has never done so. Yūki initiates a discussion of cats after seeing one earlier in the morning. After hearing Ryōta's complaints on the independence of cats, the girls try to convince him otherwise.
When Megumi misplaces her bloomers, the girls blame the boys theft and begin searching their bags and person. Chika and Ryōta bet on who's the tallest. Even though Ryouta wins in overall height, he sulks over having short legs. Ryōta and Kōji decide to have a duel to see who could win the three-legged race at the sports festival. Ryōta remembers a pinky promise he and Chika had when they were little to get married when they grew up. Chika too remembers a similar promise. Ryōta misunderstands what memory Chika recalls, however they make up in the end.
After a swimming lesson in PE class, Ryōta and Yūki struggle with washing their eyes as they are scared of water spouting from the tap they have to use. Having had to deal with Ryōta complaints of eyes stinging from the chlorine, Chika intervenes and she manages to trick Yūki into washing her eyes. However as she tries to force Ryōta to do the same, their friends cause Ryōta and Chika to have an accident. In trying to take revenge Ryōta accidentally causes Yūki to fall into the pool and in apology, he must give her his clothes so she can change out of her wet ones.