Shougo is the youngest son of the Karugano family, born and raised in his family with his four sisters. His sisters loved him since he was in his mother's womb and they looked forward to his birth; despite the fact that time has passed and Shougo has grown, for them, he is still their little brother whom they love and spoil him a lot. Although Shougo sometimes feels a little overwhelmed since his sisters can be a little overprotective. The eldest daughter Hitoha is a nurse teacher, the second daughter Niina is a physical education teacher, and the third and fourth daughters Misora and Shio (who are twins) are Shougo's seniors. Everyone goes to the same school with Shougo and lives with him in their family house. One day, Shougo was diagnosed with a strange disease which prevents his height from increasing normally, the doctor suggests that this is because while he was a baby he did not receive enough breast milk. Hitoha immediately calls a family reunion with her sisters, because their parents are not living with them. Hitoha suggests that they should supply Shougo with breast milk. "I think our little brother isn't growing well because he doesn't have enough nutrition in his childhood... but we can help him by breastfeeding him" Only the Kuragano sisters can protect Shougo in this family without parents! The four sisters got a mysterious secret drug called "Bonus Dema Cool" to breastfeed the protagonist! And from that day, Shougo began his sessions of breastfeeding with his sisters!?