Ryuuichi Kashima has always taken care of his little brother Kotarou due to their parents traveling most of the time. After the Kashimas suddenly die in a plane crash, an elderly woman named Youko Morinomiya offers to become the brothers' guardian until they both become adults. It turns out that she lost her son and daughter-in-law in the same tragedy. The teenage Ryuuichi can also attend the school where Youko works as chairwoman, on one condition: he must be part of the "babysitting club," which serves as a place to look after the children of professors.

At first, it may seem like a rough start for Ryuuichi, but with the help of his classmate Hayato Kamitani and many others, he will experience novel adventures with the children in the club.