Toshi, an ordinary high school student, spent his days being fed and taught by a beautiful neighbor named Yū-neesan, who lived near his house instead of his parents, who were rarely home. The older sister was kind and perfect as an adult. However, she had a hidden side in that her personality changed completely when it came to games… Games aren't just for fun! Big sister gets angry, gets naked and spares no means! Toshi gets caught up in the duality of her personality! Here begins a surprising new erotic comedy with games and a big sister!