You and Kasumi Otonashi are lovers.
However, due to Kasumi's position as a teacher, your relationship must be kept secret at school.
Alas, sometimes your feelings get the better of you, and you end up doing naughty stuff.
One day, Kasumi's sister, Shiori, sees the two of you having sex at school.
The shy, perverted Shiori joins in, and the three of you decide to have fun together...