Since her mother's passing Hinako and her father Keichi were everything to each other.
They lived in a neighborhood with another father and daughter named Yosuke and Shiori. Lately Hinako and Shiori played together a lot, and Keichi was beginning to have dirty thoughts about his daughter's friend.
It was around this time that Yosuke revealed he was attracted to Hinako and proposed a trade. "Let me have sex with Hinako," he said, "and I'll let you put it in my Shiori..."

[From DLsite English]