Based on the adult game by Marine.

Kobayako Ayano is frutated because her breasts do not grow as big as her teacher. He has done various ways but still not as big as his teacher's breasts. Once he knows the secret that can enlarge the breasts as big as him because of the blessing of the actions of his younger brother's teacher who every day massaged her breasts. Since then Ayano also wants to try to enlarge her breasts by asking for help on his younger brother's teacher.

(Source: Translated and Edited from Nekopoi)