Overflow is a short-form hentai anime that explores the intensely romantic and physical relationship between a young man and the two sisters who are always by his side. Kazushi Sudou lives alone, but his apartment is a second home to the Shirakawa sisters: the outspoken and fiery older sister Ayane, and the quiet, more reserved younger sister Kotone. Their long history as childhood friends creates a comfortable, yet frustratingly platonic, bond—until one night in the bath changes everything.

After a cramped bath together leads to an accidental but passionate encounter with Kotone, the dam of repressed feelings bursts. Kazushi finds himself navigating a new, secretive dynamic with both sisters. As they move from the bathroom to the kitchen and even the school locker rooms, each stolen moment and intimate act deepens their connection. Torn between his feelings for both Ayane and Kotone, Kazushi must confront the new reality of their relationship. Will this triple-bound bond bring them closer together, or will the complicated emotions cause everything to overflow?