Takashi, a part-timer, has a younger sister named Kotone, a high schooler who is quiet and reserved. One day, he casually uses a "Hypno-Ray" he borrowed from a friend on her, making her nipples 100 times more sensitive than a normal person's. Since then, Kotone's daily life has completely changed. She can't stop reacting to any nipple stimulation, often ending up with a mind-broken expression and leaking urine. Takashi can't contain his excitement at his sister's transformed state and keeps venting his pent-up sexual desires on her. He relentlessly teases her nipples every day, making her climax over and over. When it's over, he uses hypnosis to erase her memory as if nothing happened...