On a Saturday night, a late phone call suddenly rings through. It's from his younger sister, Yuu, saying, "I missed the last train, so let me stay over~." He meant to refuse, but minutes later, the doorbell rings and she walks in uninvited. She seems tipsy, standing unusually close, touching his body frequently. Her top hangs loose, and she deliberately presses her body against him. Worried she might realize his arousal, he nervously says, "Uh, you're kinda... touching it." "Hmm? This?" she replies, pressing harder. "Yeah, I'm doing it on purpose." Oh crap, this is bad.