A relative, older sister AIKA, came to stay at the boy's house for a week while he was house-sitting. While they were talking alone until her parents returned, AIKA's older sister spilled her tea and her tanned, healthy body became wet and transparent, and the feeling of fuzzy feeling she felt for the first time filled her imagination. It stirred up... AIKA's older sister noticed her delusion and seduced her in every way possible.