Kippei Katakura is the handsome playboy of the school. After ditching school to play around with a girl on the roof, his friends ask him to go play around. However, he received a phone call, telling him to go home right away. His 5 year old cousin, Yuzuyu, is staying with him, and he is the one in charge of taking care of her.

Kippei makes onigiri for Yuzuyu to take with her. He isn't very good at making it and when a boy laughs at her because of the funny looking onigiri she slaps him. Kippei tries to find out what is wrong.

Yuzu is becoming some-what of an outcast at school, because she is motherless. She has Kippei though, so she'll be okay, but what happens when a seriously Kippei-crazed girl shows up?

Yuzu is left alone and Kippei's stalker comes up and talks to her. Stalker says that everybody hates her and throws away Yuzu's crayons! Now Yuzu, feeling very insecure, tries everything she can to keep Kippei by her side.

Yuzuyu is asleep for the night. Kippei is still thinking about why Yuzuyu wouldn't tell him what happened to her crayons. The next morning Kippei promised to pick up Yuzuyu right after school. On the way he runs into this old lady that needs help with directions. While Yuzuyu's waiting the girl who got rid of her crayons comes to pick on her some more. Will Kippei make it in time?

At school today Yuzuyu is playing in the sand. Her classmate, Marika, comes by and says that Yuzuyu is a show off, because her uniform is different than everyone else. Reiko and Kippei to the store and buy her a new uniform that matches. Will she wear it?

A letter is received from Sakamoto Miyako, Yu's mother, Kippei's family stating she still isn't coming home, along with that letter there is one for Yu as well. Kippei gives the letter to Yu without looking at it, leaving him curious as to what it says. Class seats are changed and Kippei ends up sitting next to Kokoro. A large portion of the episode revolves around Yu's stuffed bear and the memories surrounding it between her and her mother.

Kokoro is going to live alone in a luxurious apartment. When Kippei and Yu are returning home from the market they saw Kokoro living alone. At the end of the episode, Kippei knows everything about Kokoro and they kiss.

Kippei is late to picking up Yu from school and gets in trouble by his family for it. Kippei and Kokoro pick up Yu from school, they go to the park where Kippei overhears a conversation between Yu and Kokoro, in which he finds out how Kokoro supposedly feels for him. Later on Marika runs to Yu wanting to know who Kokoro is, Yu tells her teacher she plans to walk home on her own from then on because Kippei wants to be with Kokoro, and Yu thinks she is getting in his way.

Kippei goes looking for Yu who went to walk home alone. Yu ends up following a puppy and decides to help the puppy look for its mother. Kippei calls his family to report he hasn't found her and they set out to search, close to nightfall. Eventually he runs right into Kokoro rounding a corner, he tells her Yu disappeared, and she helps him look for her. Yu realizes she doesn't know where she is and gets scared. He finds her late at night and she still claims to be able to get home alone. After a short talk he learns why she has been acting like she has. Kippei and Kokoro break up and Kippei picks up the suddenly bashful Yu.

Yu doesn't have school today, so she makes a pocket lunch for Kippei and delivers it herself. Some of the female high schoolers upon seeing her seem to like him more; some of Kokoro's true feelings show.

Yu's mother shows up outside the kindergarten multiple times in one day; the teachers thinks that she's a suspicious person. Kippei is asked to get rid of the suspicious person only to find out who it is. He is asked by Yu's mother to not tell anyone that she was there, especially Yu.

Yu meets a boy named Shouta at school and likes him from the start. Natsu tries to get Kippei to go out with her so she can buy stuff for cheap and gets rejected, later she finds out she loves him. It hinted that Shouta has been hit by his mother.

Yu brings home a slip telling Kippei of Parents Day, but he can't go because he has school, even though Yu has been working on her dance, so Kippei's sister tells him to skip and says she'll go with him. Throughout the episode it shows bits of Shouta's family life. After Yu sees another bruise on Shouta, and Shouta's lie about how he gets it both Yu and Kippei decide to figure out what's happening to Shouta. Kippei see's Shouta's mother at the parents day. Yu dances in her class, and Shouta makes a sword of paper instead of flower in his, making his mother angry. Yu see's Shouta being hit by his mother and starts crying bringing everyone out to see, an argument between Kippei and Shouta's mother happens soon after. Shouta is abandoned by his mother but runs after her.

Kippei is down because of Shouta's mother's words. Kokoro's true feelings are shown again. Kippei confronts Shouta's mom after school. Shouta's and his mothers life is shown some more, Shouta is sent to the hospital after being slapped and then falling down stairs, also some of Shouta and his mother's past is shown. It is hinted that Shouta's family is going to go to counseling. Also at the end Shouta's family states they are moving back to the suburbs, then leaves after a heart felt good-bye between Shouta and Yu.

Yu's down because Shouta moved away. It is found out that Yu is scared of the pool, so Kippei decides to take her to the pool so she can get over her fear. Kokoro comes to the pool as well and Kippei falls in love with her bathing suit, though he doesn't think she should wear it in public. Kokoro helps Kippei get Yu to go under water.

Miki comes to Kippei's house saying her family wants to take Yu. Kippei's family discuss it and his sister decides to contact Miki's family. Miki's family go to Kippei's house angrily, since they never sent Miki there. She runs away and Kippei goes to find her, when he does he finds out she has purposefully hurt herself a number of times.

Miki states she wants to die which is why she wants Yu to go to her house instead of herself. Kippei gives Miki a taste of life in his house under his care. Bits of Miki's past is shown, and she follows Kippei to "Go to School Day". Miki sees Kippei kissing Kokoro and changes back to her old mean self. Miki heads home and tricks Yu into leaving with her.

Miki takes Yu to her house as Kippei and his family wait, hoping that Miki hadn't stolen Yu away. Kippei goes to pick them up after Miki tells him where they are, during the time it takes him to get there more of Miki's past is revealed. When Kippei gets there Miki says she's going to kill herself and he talks her down. Later Miki and her parents make up.

Kokoro is mad at Kippei for not calling her even once over the summer like he has promised to do. Kokoro's friends tell Kippei that Kokoro is popular with the guys so he had better start acting like a real boyfriend or Kokoro would be swept off her feet by another man. Miki comes back for her things, now a happy girl, and watched Kokoro before she leaves to go home.

Kippei catches a cold. A model in Satsuki's class likes him. A little of Kippei's childhood is shown as his family tells Yu of it. Satsuki's model friend is told by other girls he doesn't like tall girls, girls with long hair, or models, which breaks her heart. Later she tells him she likes him.

Kippei makes two box lunches, one for Yu and one for Kokoro. Kokoro gives her homemade lunch to Kippei in return for him making one for her. During eating she gets a phone call that depresses her. Marika gets picked on by one of her friends at class but Yu tells her she's still her friend and that she likes her. Kokoro tells Kippei she doesn't want to get any closer to him.

Yu's letter cheers Kippei up and Kippei decides to write a letter to Kokoro so she can feel his feelings more. Kokoro doesn't show up at school the next day, and some of her past with her mother is shown.

Yu has a dream and realizes she doesn't remember what her mother's face looks like and starts to miss her mother. Kippei accidentally tears Yu's pajamas, which were made by her mother. Kokoro and Yu spend some time alone with each other. Kippei's sister flies away from home.

Kokoro and Yu are still in the sandbox as they were in the last episode. Kokoro cheers Yu up. Kippei's family finally meets Kokoro and invites her to stay the night with them. Kippei's sister meets with Yu's mother. Kippei gives Yu her pajamas back all fixed up. Yu's mom and Kippei's sister talk about Yu and the problem with her pajamas. Yu's mom talks about the past and then Kippei's sister leaves for home. Kippei, Yu, and Kokoro sleep together.