"My Classmate's Mother" centers on the life of a high school student, Yuuto, whose ordinary routine is disrupted when he becomes entangled in a complex relationship with his classmate's mother, Akiko. Yuuto initially interacts with Akiko through casual encounters, but their connection deepens as they spend more time together. Akiko, a widow struggling with loneliness and the pressures of single parenthood, finds solace in Yuuto's company, while Yuuto, navigating the challenges of adolescence, is drawn to Akiko's maturity and understanding.

The narrative explores the evolving dynamics between Yuuto and Akiko, highlighting the emotional and ethical complexities of their relationship. As their bond grows, they must confront societal expectations, personal insecurities, and the potential consequences of their actions. The story also delves into the perspectives of other characters, including Yuuto's classmate and Akiko's daughter, who becomes increasingly suspicious of their interactions.

Themes of love, responsibility, and moral ambiguity are woven throughout the series, as the characters grapple with their feelings and the impact of their choices on those around them. The plot unfolds with a focus on character development, portraying the internal struggles and growth of both Yuuto and Akiko as they navigate their unconventional relationship. The series balances moments of tenderness and tension, creating a nuanced portrayal of human emotions and relationships.