"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 their parents remarried, making them step-siblings. The story explores their awkward coexistence as they navigate their new family dynamic while grappling with unresolved feelings from their past relationship.

Mizuto is a reserved and introspective boy who prefers to keep his emotions in check, while Yume is more expressive and prideful, often masking her vulnerabilities with a confident demeanor. Their shared history as ex-lovers adds tension to their interactions, as they frequently clash over their differing personalities and lingering emotions. Despite their attempts to maintain a platonic relationship, their proximity and shared experiences often lead to moments of vulnerability and introspection.

The narrative delves into their daily lives, highlighting the challenges of living under the same roof while attending the same school. Their interactions are marked by a mix of humor, frustration, and occasional tenderness, as they struggle to reconcile their past with their present circumstances. Supporting characters, such as their classmates and family members, provide additional context and influence their evolving relationship.

The story balances comedic elements with emotional depth, exploring themes of love, identity, and personal growth. As Mizuto and Yume confront their feelings and the complexities of their situation, they gradually come to understand themselves and each other better. The series captures the nuances of adolescent relationships and the impact of family dynamics on personal connections, offering a relatable and engaging portrayal of young love and self-discovery.