"My Stepmom's Daughter Is My Ex" centers on the complicated relationship between Mizuto Irido and Yume Ayai, two high school students who were once romantically involved but broke up before entering high school. Their lives take an unexpected turn when their parents remarry, making them step-siblings and forcing them to live under the same roof. The series explores their awkward and often humorous attempts to navigate their new family dynamic while dealing with unresolved feelings from their past relationship.

Mizuto is a reserved and pragmatic individual who prefers to keep his emotions in check, while Yume is more expressive and struggles to reconcile her lingering feelings for Mizuto with her pride. Both characters are acutely aware of their shared history, which adds tension to their interactions. Despite their efforts to maintain a detached and civil relationship, their proximity and the remnants of their past romance create moments of vulnerability and introspection.

The narrative unfolds through a mix of slice-of-life scenarios and comedic misunderstandings, often highlighting the contrast between their public personas and private emotions. Secondary characters, such as their classmates and friends, occasionally provide additional perspectives or contribute to the comedic and dramatic elements of the story. The series balances lighthearted moments with deeper explorations of themes like maturity, communication, and the complexities of human relationships.

As the story progresses, Mizuto and Yume gradually confront their unresolved feelings and the reasons behind their breakup. Their evolving relationship is marked by a blend of awkwardness, nostalgia, and tentative steps toward understanding each other better. The series captures the nuances of their dynamic, portraying the challenges of moving forward while acknowledging the past. Through its focus on character interactions and emotional growth, "My Stepmom's Daughter Is My Ex" offers a thoughtful and engaging portrayal of young love and familial bonds.