A tradition passed down in the busty family... if a son is born, the mother will use her busty breasts to take the virginity of the son. Having learned this tradition from their mother Fumie, busty sisters Naho and Rin each struggle to take their son's virginity, but they struggle... One day, the sisters take their sons and return to their parents' home for the first time in a long while. There, Fumie hears that her grandchildren are still virgins, and decides to perform a certain ritual. It involves Naho engaging in obscene acts with Rin's son, and Rin engaging in obscene acts with Naho's son...