Through the power of a memory alteration spell, Rurumo begins living with the Shibaki family under the guise of Kota's younger sister. Despite them living together, Kota experiences great distress at his inability to do anything to his "younger sister". Meanwhile, in order to punish herself for having too much fun, Rurumo begins working in the cafeteria at Kota's high school, where she makes mistake after mistake.