Three guys had been targeting Amu, a fellow member of their music club, for some time, thinking that she had big breasts and would probably do it with them if they asked. So on the way home from a club drinking party, a drunk Amu was invited to their house for a drink, and she enthusiastically came along, so they got her drunk, played some erotic games to get her excited, and tried to gauge her reaction by touching her body, which seemed to please them, so they were happy to touch her as much as they wanted, and so they all had sex.