My Stepmom's Daughter Is My Ex - 1 ª Season

Yume and Mizuto become stepsiblings after Yume's mother Yuni married Mizuto's father Mineaki. Unfortunately, their parents are unaware they once dated in middle school.

Mizuto is approached by Kogure Kawanami, who states he will be Mizuto's friend and has no intention of using him to get to Yume.

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.