When summer vacation begins, the niece sisters visit their uncle's house every day. Energetic and full of curiosity, they play around wearing school swimsuits, proudly showing off their sun-tanned skin. The older sister, "Chiharu," has always been mischievous and matured early, gradually becoming more flirtatious toward her uncle. Meanwhile, the younger sister, "Ruru," starts drawing his attention as she blossoms. One day, the uncle accidentally teases Ruru, sparking jealousy in Chiharu, who then intensifies her advances. Driven by the fear of losing their uncle, the sisters compete—Chiharu desperate to keep him, and Ruru imitating her sister to get closer. As both niece sisters passionately pursue their uncle at the same time, how will he respond?