Based on a light novel by Aoyama Sagu.

High school student Hasegawa Subaru decides to stop associating himself with basketball, despite his love for the sport, when his school's team captain was suspected being a pedophill and the club is disbanded for a year. His aunt, Takamura Mihoshi, then assigns him to be an elementary school girl's basketball team coach. Initially, he accepts to train the girls just for three days, but after learning of their circumstances, he decided to keep coaching them.

[Edited from Wikipedia]