Masamune likes his older sister, Minaho. But she has gone abroad to study soon after he entered a high school. He really likes her, her smile and her big and soft breasts.... "I wanna touch them!" He always feels down at the classroom and Momoko, his old friend, tries to cheer him up. At first, he doesn't see Momoko as a girl but when he takes a good look at her, he finds out that her breasts have been getting bigger since she entered the high school. One day, a fill-in teacher comes to his class. It is his older sister, Minaho! His life full of H (ecchi) starts....