Kazumi approaches Ryōta to request his accompaniment for some after-school tasks in order to dislodge a loose but recalcitrant baby tooth. One day after school in front of a convenience store, Ryōta avails his newly-won bright-red superball in a duel with Kouji's slightly-larger blue superball to see whose superball can bounce higher. During one afternoon in the absence of adult supervision, the students of class 5-2 soon abandon cleaning their classroom and begin simulating a rock concert using brooms as air guitars, much to Yūki's disappointment. Ryōta, Kōji, and Tsubasa forget their umbrellas on a rainy afternoon and ask Chika and Natsumi to help them out. However due to his size, Ryōta is unable to walk home with Kōji and Tsubasa and is invited to instead walk with Chika and Natsumi.
While en route to school, Yūki encounters a playful cat that follows her to school. Ryōta does not understand what Yūki and her friends find so fascinating about felines. When they try to explain, the cat that followed Yūki to school unfortunately scratches his face. During recess, after scoring a football goal against Ryōta, Natsumi compels him to help her retrieve some plastic cones from the storage shed. While two search the inside for the cones that are lie beside the shed unnoticed, the PE teacher locks them in believing the shed to be empty. Taking advantage of Natsumi's nyctophobia, Ryōta engages in various antics to exact revenge for his earlier defeat to his later regret. As P.E. class draws to a close, Ryōta accidentally wanders into the midst of the girls changing in order to retrieve a football and is subsequently beaten. In the next class, Megumi is at a loss for where her gym bloomers could be. After jumping to the conclusion that the thief is one of their classmates, the girls initiate a search of all the boys' bags and person for the missing bloomers.
Ryōta claims that collarbones are the best female corporeal quality in order to divert attention from himself when the boys of the class demand that he justify to the girls why guys like gravure magazines. At the end of school in the park, the girls and boys gather in their own individual groups to reflect on the matter. After Ryōta and Kōji are dragged to the library by Tsukasa, they encounter Chika who requests Ryōta's shoulders for a vertical boost. Because she fools around, Ryōta loses his balance and the two crash on to Kazumi. Seeing Kōji use a chair as an aid, the two launch into an argument over who is to blame for their accident. A pair of underclassmen boys rushing by to flip Yūki and Chika's skirts prompts the question of when children begin to exceed the age limit for excusing that kind of behavior. When they ask their friends, they receive many different and end up finding out to their disbelief that Ryōta has never flipped a girl's skirts before. Later, Chika decides to play a prank on Kōji by accidentally leaving a recently-purchased skirt on his desk. After the incident is resolved, Kōji suggests that they attempt this prank on Ryōta.
At the conclusion of a watery P.E. lesson, Chika and Yūki find Ryōta struggling to wash the chlorine from his eyes as he is scared of the water burst. Having had to deal with Ryōta's resulting complaints, Chika attempts to intervene until their friends interfere. Later in class during lunch, Ryōta accidentally empties a carton of milk on Megumi and drops his overfilled soup bowl on Yūki. On his way to the nurse's office to apologize, he instead discovers Natsumi sleeping after being stripped by Megumi and Yūki for a change of clothes and is subsequently beaten by the two when they return. At the end of the school day, Ryōta and Natsumi argue over which is the correct method for taking off one's shirt, from the top or from the bottom respectively. While answering Kōji's question on their relationship, Ryōta emphatically declares that he and Chika are only childhood friends. Anguished upon hearing him say so, Chika unsuccessfully tries to trick Ryōta into kissing her.
Ryouta, Kouji, Tsubasa, and Chika each mount an unsuccessful attempt to challenge the history of never having seen Kazumi laughing. At night while waiting for a meteor shower, Ryōta falls asleep and demonstrates his ability to force people to fall asleep. Before a swimming class, having forgotten her modesty poncho, Megumi is at loss as to how to change into her swimming suit until Yūki shows her the technique for changing without it. Having forgotten his modesty toga and wondering how to best put on his swimming trunks, Ryōta challenges Kouji to a contest of who can change from underwear into trunks the fastest.
Yūki suggests Kazumi observe a more dynamic event than flower growth - experimenting with different methods to wake Ryōta from his nap in spite of his earlier warning to do otherwise. After school one day, a game of dodgeball of boys against girls is interrupted by a sudden rainstorm that forces the children to take cover. However, they decide to continue to play; even Kazumi joins in. In the summer heat, Ryōta is challenged to several several games of rock-paper-scissors with the penalty of fanning the winner. Ryouta's attempts to defy his constant losing streak land him in hot water with Chika who promptly beats him up for accidentally flipping her skirt. Ryōta, Kōji, and Tsubasa rush to the summer festival but arrive too early. Natsumi, Kazumi, Megumi, Chika, and Yūki arrive shortly thereafter dressed in their yukatas and both the boys and girls began to enjoy the festival together.
After Natsumi successfully blocks his football goal, Ryōta begins to frantically challenge her to various athletic activities, betting all the boys' lunch desserts. After facing defeat at every turn, Ryōta finally finds something he is better at -- embroidery. On the way back from school, Ryōta and Kōji decide to have a water gun fight, battling each other as leaders of their own individual teams. All proceeds smoothly until they soak the passing group of girls from their class with an errant blast of water. Out for revenge, the incensed girls march to the store to buy their own water guns and utterly defeat both teams of boys. While in front of a bookstore, Megumi debates over buying one of two volumes of manga she wants but solves her dilemma when she decides to share the cost of the two volumes with Kazumi. In the same store, Ryōta's debate on whether or not he will buy magazine meant for young kids containing an action series he wants is resolved toward buying it after seeing Tsukasa do the same. Challenged by Kōji who is partnered with Chika to who would win the school's sports festival event, the three-legged race, Ryōta practices obsessively with his partner Yūki only to end up in last place.
Megumi stares at her uneaten lunch one day due to worries about her own weight. Out of concern, Ryōta investigates and discovers that the scale in the nurses office is improperly calibrated. While still dieting, Megumi's willpower is put to the test when Kazumi decides to record Megumi's reaction in her observation diary to various foods as well as experimenting to see if pain or laughter can displace hunger. At the end of class, Chika and Megumi decide to compare heights and find out that Chika has a slight height advantage. Because it was mentioned, Natsumi and Ryōta begin to worry about their own heights. To spite Ryōta, Kōji proposes that he compare heights with Chika ending in stalemate between Ryōta's win in overall height and Chika's longer legs. Natsumi begins to binge on milk in an attempt to develop her breasts, eliciting the awe of her classmates due to the volume and speed she can intake. This prompts Ryōta, goaded by Kōji, to engage in a milk-drinking contest which he loses.
The students of Class 5-2 attentively watch the TV broadcast on a nearby typhoon in hopes that it will pass over them and cancel afternoon classes, however the storm changes course. One day when Kōji is sick from a cold, Yūki comes to school dressed nicely after having visited a beauty salon looking for praise from her classmates. After school in the soporific summer heat, Yūki suggests to the boys that everybody play dodgeball together so as to stay awake. While playing a game, Kazumi twists her ankle by falling asleep standing. Unfortunately, Ryouta is made to piggyback transport home the sleeping Kazumi whose sleep talking earns him a glower of jealousy and a headbutt from Chika. Later during a home economics class, Yūki demonstrates her terrible culinary skills.
The cold weather and Natsumi's complaint of how hot she feels prompts several of the girls in the class to place their cold hands on her face. Ryōta's disparaging remarks about the girls' conduct diverts their reaching hands to him only to find that all their hands except for Kazumi's have already warmed up. Saliva sterilizing wounds seems to be a prevalent superstition throughout the grades. Yūki tries to get Chika to kiss Ryōta in order to heal his irritated eye, which results in Chika punching him. Some time later, Yūki finds Kazumi writing a fairy tale featuring all the main characters with Ryōta as a prince cursed with short legs and Chika as the princess about how the prince must goes through several trials in order to save the princess. One day, Kōji and Tsubasa rush to Ryōta for assistance in helping Natsumi deal with the morbid conclusion of a book she just read.
The dormant classroom heater prompts Yūki, Kazumi, Megumi, Natsumi, and Chika into the health office to warm up and then to begin measuring each other's corporeal dimensions after chasing the school nurse out. Ryōta is twice violently repelled by the girls from resting in the nurse's office due his deteriorating respiratory ailment. After school, some of the students of Class 5-2 discover a lone duck swimming in the school pool whom Kazumi names Kobayashi. When their attempts to send Kobayashi along with a flock of ducks flying overhead all fail and they began discussing how to take care of it, Koyabashi gets its own volition to fly. Yūki, Megumi, Haruka, and Chika interpret Kazumi coming to school one day with her hair in pigtails as license to stylize their own hair that initiates a competition between the four of them that lasts for over a week. Ryōta, Kouji, and Tsubasa engaging in their winter ritual of stomping the ice, before comparing for the biggest piece and then playing with it earns them an austere reprimand from Chika and Natsumi.
Yūki, Kazumi, Megumi, Natsumi, and Chika arrive at a store selling roasted sweet potatoes while walking down the street on a cold breezy day. Ryōta, Kōji, and Tsubasa's sudden appearance interrupts the girls' silent competition over a quintet of sweet potatoes spanning a spectrum of different sizes. During school, it begins to snow. While waiting for the snow to accumulate, the students of Class 5-2 become involved in a debate over the meaning of a childhood nursery causing them to be the last outside. Later in town, Ryōta, Kazumi, test their luck at the district lottery. After home economics class, Yuki tells Natsumi that chalk is edible. Chika stops her just in time and decides to see if Ryōta would fall for the lie. Ryouta poorly handles the deception by lashing out at Chika's cooking to his regret. Fortunately after a few days, Ryouta manages to win Chika's forgiveness by kissing her.
Ryōta dreams of a scene from his past involving a promise of marriage between him and Chika before waking up late and dashing to school. However, his euphoric morning is quickly extinguished when Yūki spreads the news of Chika having dreamt that same morning of a promise she made to Ryouta after he wet the bed when in kindergarten. Ryōta lashes out at Kazumi pitying him for wetting his bed under the impression that everyone was talking about Chika remembering their promise to marry each other only to realize his mistake when Chika apologizes to him the next day. Ryouta begins to sulk until Chika approaching him alone at the end of the school day to cheer him up causes him to recall the other promise that he made with Chika after wetting his bed in kindergarten: to try his best to become a better man. Ryouta catching Chika as she is about to lose her balance while changing her shoes signals his decision to forgive her. Back from winter vacation, Chika begins wearing the hairpins that Ryōta gave her as present.