Yamada, a freelancer obsessed with asses, had his niece rina-chan stay over. Rina-chan, who only plays games all day without listening to anyone, carelessly exposes her beautiful ass. Unable to resist, Yamada buries his face in rina-chan's ass. At first surprised by the sudden act, she eventually enjoys it and starts demanding more, reversing their roles. From that day forward, their secret relationship began.