The joys of summer break; warm weather, sun soaked beaches, time off from school, and in the case of Yuuta Yamauchi, a torrid love affair with your cousin. Summer Sis details the sexual relationship between Yuuta and his cousin, Kei Higashide, who offered to help him study for his college entrance exam.

As their relationship develops, Kei becomes aware of a girl named Youko who may be toying with Yuuta. Yuuta's summer is about to get very complicated, and it's anybody's guess if he'll actually get any studying done!