My sister, Mitsuki, is a middle school student. Lately, as she's gradually becoming aware of her sexuality, she's been acting flirtatious toward me constantly, day and night. Especially recently, she starts teasing me the moment she wakes up, pushing me closer and closer to my limit. On top of that, our parents have just left for a two-day, one-night hot spring trip, leaving the house completely empty. For Mitsuki, this is the perfect environment. In this situation, her actions toward me continue to escalate further and further.