Masato's parents passed away two years ago and he now lives with Saki, his older sister. Saki is a teacher and takes a good care of him. She is always kind to him and he gradually falls in love with her.... One day, when he gets home, he hears someone having a shower. Yes, it's Saki and he finds her underwear in the laundry basket. He can't stop his sexual desire and starts masturbating using it. However, the bathroom door suddenly opens and Masato runs away into his room.... A while later, Saki enters his room without saying a word. She is naked and suddenly pushes him down on the bed....