When a guy heads to a mixer party at a friend's invitation, he is shocked to see his real little sister Mina there. Both decide to hide their sibling relationship and join the fun. But things heat up fast when the group starts playing King's Game — and every command turns outrageously sexual: flashing breasts, showing panties, the Pocky game, and even forced masturbation displays. Unable to reveal the truth, the brother and sister are pushed further and further until they cross the ultimate forbidden line.

In the second episode, the protagonist discovers a mysterious hole in the wall between his room and his sister Sayaka's. Peeking through reveals Sayaka secretly masturbating. The next day he bravely inserts his cock into the hole, and Sayaka treats it like a fun "mushroom" game. What begins as anonymous teasing quickly turns into raw, passionate sex as the shy sister finally gets the creampie she's always secretly wanted from her big brother.

Full of intense incest, risky situations, and nonstop erotic action, this OVA is pure guilty pleasure for fans of little sister (imouto) hentai.

Based on the CG collections by Yorokobi no Kuni (ヨロコビの国).