Kyosuke never liked his step-mom and wants her out of the house after his dad died. Megumi proposes that Kyosuke can do anything he wants with her daughter, Yuna, if he allows them to stay. He allows them to stay.
Schoolboy finds out that his girlfriend has been secretly masturbating in public parks. He decides to help her out and turn that addiction into public fuck sessions for the both of them. Win-win.