"Overflow" is an anime series that centers around the lives of three main characters: Kazushi Sudou, his younger sister Ayane Sudou, and their childhood friend Kotone Shirakawa. The story begins with Kazushi, a university student, moving into a new apartment. His sister Ayane, who is overly attached to him, and Kotone, who harbors feelings for him, both decide to stay with him temporarily. This living arrangement sets the stage for a series of intimate and complex interactions among the trio.

Kazushi is portrayed as a kind and somewhat passive individual, often caught between the affections of Ayane and Kotone. Ayane, despite being his sister, exhibits a strong romantic and possessive attachment to Kazushi, which creates tension in their relationship. Kotone, on the other hand, is more reserved but equally determined to express her feelings for Kazushi. The dynamic between the three characters is further complicated by their shared history and the emotional baggage they carry from their past.

The narrative explores themes of love, desire, and familial bonds, often blurring the lines between them. The interactions between Kazushi, Ayane, and Kotone are marked by moments of vulnerability, jealousy, and longing, as they navigate their feelings for one another. The series delves into the complexities of their relationships, highlighting the challenges they face in reconciling their emotions with societal norms and personal boundaries.

Throughout the series, the characters' relationships evolve, with each episode revealing new layers to their connections. The story does not shy away from depicting the physical and emotional intimacy between the characters, often using these moments to underscore the depth of their feelings and the conflicts they experience. The series maintains a focus on the interplay between the characters, with the apartment serving as a confined space where their emotions and desires come to the forefront.

"Overflow" is characterized by its exploration of unconventional relationships and the emotional turmoil that accompanies them. The series presents a candid portrayal of the characters' struggles, offering a nuanced look at the complexities of human connections. The narrative is driven by the interactions between Kazushi, Ayane, and Kotone, with each character's motivations and emotions playing a crucial role in the unfolding of the story.