Brother and sister living in a traditional Showa-era Japanese house. Four years have passed since they lost their parents, and their younger sister Nanami has stopped speaking. The older brother, Masashi, deeply cares for her and acts as her guardian. On a hot, humid summer day, the two enjoy daily cool baths together in a tub filled with soap bubbles. Their innocent routine is warmly observed by neighbors. While Nanami's mind remains frozen in childhood, her body matures rapidly. Like the "Seven Seas," quietly yet powerfully, slowly but surely, she transforms from girl to woman. One day, Ichiro, Masashi's childhood friend who runs a liquor shop, brings over some used yukata. Nanami happily accepts them with a childish smile. From that moment on, changes begin to emerge in her body. Her womanly form has already reached its peak, and natural desires toward men begin to awaken. Ironically, the only people around are her devoted brother and Ichiro, who watches over them both. With a childlike heart and a fully matured body, she begins to follow her instincts without restraint.