Mizuto y Yume solían salir juntos, pero ahora se han convertido en hermanastros. Ambos están de acuerdo en que no quieren hacer nada que decepcione a sus padres recién casados. Sin embargo, ninguno de los dos puede aceptar que el otro sea el hermano mayor de la familia. Así que ambos acuerdan resolver el asunto con un juego. El que haga algo que entre en conflicto con lo que la gente considera un comportamiento normal de los hermanos, pierde. De esta forma, el hermano menor se convierte en el mayor.

Yume enferma y tiene que quedarse en casa justo después de hacer una nueva amiga, así que esta decidirá ir a visitarla a casa... para fastidio de Mizuto.

Cuando Yume se da cuenta de que se ha llevado a su cuarto la ropa interior de su hermano para olerla, se da cuenta de que su vieja forma de ser está saliendo a la luz sin que ella lo quiera. Tendrá que armar una operación para dejarlos en su sitio sin que la descubran.

As punishment for the underwear incident, Mizuto demands Yume go on a date with him. Yume is taken aback by the sudden development, but little did she know that it was all part of Kawanami Kogure's devious plan, a plan to shatter Minami Akatsuki's delusional dreams of marrying Mizuto only to become Yume's doting little sister. And how else to do it but to show her that Yume only has eyes for Mizuto?

It's Mother's Day, so Yume and Mizuto go out to buy a gift together. Their mother, Yuni, is overcome with happiness to receive a gift from the two of them. Seeing how happy it made her, Mizuto wonders if it'd be nice to give their parents some time to be newlyweds together. However, there isn't much they can do for them on a high schooler's budget. So Mizuto suggests that he and Yume spend the night somewhere instead.

The first midterms of the school year are upon them. Once the top scoring student in the school's entrance exam and now a talented student at the top of her year, Yume is anxious to defend her position as a model student. She is displeased, however, to discover Mizuto’s relaxed approach to studying. The two later butt heads again, over something Mizuto did during their Japanese exam. Yume berates Mizuto for his actions while Mizuto retaliates in kind.

Due to their mutual interest in light novels, Isana considered Mizuto a dear friend. However, Yume and Akatsuki make her realize that her feelings for him may run much deeper than that. Now aware of her true feelings, Isana wishes to become Mizuto's girlfriend. Unfortunately, he doesn't seem to be reacting to her advances. Yume and Akatsuki try to give her advice, but it doesn't quite go according to plan.

Isana is coming over to the Irido house this weekend to hang out with Mizuto. Suspecting that she may not know how to properly dress for the occasion, Yume and Akatsuki invite Isana out to go clothes shopping the day before. Unfortunately, however, every outfit she tries on ends up looking inappropriate due to the size of her breasts. Akatsuki suggests that she try on something with a softer silhouette similar to Yume's usual style, seeing this as an opportunity to pick out new clothes for Yume to try on too. Yume then reminisces about a date she had in middle school.

It's Yume and Mizuto's first summer together as stepsiblings. Their parents are unable to return home one night due to a typhoon. Now alone in the house together, trouble arises when Yume spots a certain creepy crawly in her room. On another day, Mizuto accompanies Yume clothes shopping for a swimsuit in preparation for an upcoming family trip to the countryside. It's in situations like these that the two seem to be finding themselves reminiscing about the past more and more...

La familia Irido ha vuelto al campo. Allí, Yume le permite leer una novela autoeditada escrita por el bisabuelo de Mizuto. Después de leer la novela, Yume de repente tiene la oportunidad de preguntarle a Mizuto sobre su primer amor. Cuando Yume escucha de Mizuto que solía reírse mucho, se convence de que la otra persona es Madoka, su prima.

Still in the countryside visiting relatives, Yume and Mizuto head to the local summer festival together. In high spirits from getting to see him in his yukata and seeing how Madoka and Chikuma interact as siblings, Yume boldly takes his hand in hers as they walk around the festival stalls. During their outing, Yume begins to get a picture of how Mizuto was as a child. And in doing so, realizes just what his meeting her meant to him.