"I" was staying over at my girlfriend's house today. I was sleeping with her in her room, but suddenly woke up in the middle of the night. "I'm thirsty..." I slipped out of bed and headed to the kitchen. As I was taking water from the fridge and drinking it, I heard some lewd voices coming from the living room. "Hmm? What's that?" I went to check on her, and found her sister tying herself up with hemp rope. Then, her sister started seducing me by showing me her masochistic masturbation, so I started training her...