Go and his sister Riho are staying with their older brother who lives alone in Tokyo to attend university there. Riho is a stepdaughter from Go's second marriage, so they are not related by blood. Riho has feelings for Go that go beyond those of a brother, and whenever she gets the chance she tries to seduce him sexually, but Go holds back and doesn't cross the line. However, his brother is turned on by Riho's huge breasts and he can't control his arousal...