Ryūichi and Kotarō Kashima are brothers. Ryūichi is in high school and Kotarō is still a toddler. After their parents are killed in a plane crash, the two brothers find themselves living with the chairwoman of the Morinomiya Academy, who herself lost her son and daughter-in-law in the same accident. The chairwoman, however, has one stipulation for the brothers in order to live with her: Ryūichi has to work at the academy's daycare center as a babysitter.