At the prestigious private dormitory school, Rikkamon Girls' Academy, there exists a partner system called the 'Buddy System,' which is established by exchanging spare keys to individual dorm rooms with someone else to help each other. The center of attention among the students is this system and the 'tea party hosted by five ladies,' who are the objects of admiration and order for the entire student body. However, the arrival of 'a certain key' to first-year high school student Kewaki Emi brings about an unexpected turn of events...