My Stepmom's Daughter Is My Ex

Mizuto and Yume used to date, but now they've suddenly become stepsiblings. Both of them agree that they don't want to do anything that'd disappoint their newly remarried parents. However, neither of them can accept the other as being the eldest sibling in the relationship. So they both agree to settle the matter with a game. Whoever does something in conflict with what people might consider normal sibling behavior, loses. Making them the younger sibling.

After entering high school, Yume was making an effort to change her personality and looks in an attempt to reinvent herself as a model student. It was all so far so good, but there was one thing she couldn't change about herself. She was never very athletic. On the day of the physical fitness test and body measurement check at school, a melancholy Yume is approached by Minami Akatsuki, a girl from the same class. Meanwhile, Mizuto is as disinterested as ever...

It's nighttime at the Irido residence. Yume has somehow found herself with Mizuto's boxers in her possession. If Mizuto were to find out, it would almost certainly mean she would have to face the punishment of admitting to being the younger sibling. Hoping to avoid this outcome, Yume waits for Mizuto to fall asleep before heading to the bathroom to return his underwear to the laundry basket. But who does she pass in the hallway, but a bewildered Mizuto, mouth agape, with Yume's very own bra dangling from his grasp.

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.

We are brought back in time to when Yume and Mizuto were once a couple. A time when neither of them could control their nerves in what was their first ever relationship. Having already held hands and kissed before, the young couple are eager to take the next step. Spring break passes, however, with neither able to make the first move. Upon entering their third and final year of middle school, the two are regrettably put into separate classes. Alas, unable to spend as much time together as before, they both begin to grow apart.

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...

The Irido family go to the countryside to visit relatives. There, Mizuto shows Yume the first book he ever read on his own, his great-grandfather's autobiography. Reading it prompts Yume to ask him what kind of person his first love was. The description he gives, that she was someone who loved to laugh, leads her to believe his first love must have been none other than his second cousin, Madoka.