Otona Yuri Gakuen, an all-girl school, has a tradition where the students treat each other as sisters. Seniors are called Onee-sama instead of being addressed as senpai and, of course, imouto instead of kouhai.

This is the story of a pair of sisters,the younger Yuki and the elder Shiori. They are dorm-mates and spend their school live normally like most students usually do. But all of these are about to change... and it all starts with a kiss.