While visiting my friend's house, he stepped out to the convenience store, leaving me waiting in the living room. His younger sister approached me and handed me a vibrator, saying, "Is this thing working? It seems to be broken." She asked me to touch it and check. I assumed it was a massage device, but when I turned it on, it worked perfectly. I told her it seemed to have just been a loose connection. She happily replied, "I have terrible shoulder stiffness and can't live without this." But as I started massaging her shoulders, the device unexpectedly began stimulating her large breasts, making her start feeling aroused right there. Before I knew it, the vibrator had moved down to her crotch. Confused and unsure of what to do, I found myself helplessly drawn into the situation.