Yuji Kashima is a boy who's very shy and has a weak personality. He came across a salesman who gave Yuji a boardgame to take home free of charge. The game is called Sōkan Yūgi (The Incestuous Game) which the salesman promises Yuji it will bring his relationship with his family "closer". Yuji asks his mother Aoi not knowing what will happen to play the game with him. As they start playing the card first says embrace. Aoi hugs him tightly after which the card reads kiss. Aoi asks Yuji to kiss her on her cheeks but he takes more pleasure in kiss her lips. And then they start licking each others tongues. Later they have sex. Then Yuji has sex with his sister.
- Yuji Kashima
Yuji (鹿島ゆうじ Kashiima Yūji) is the main character of the story. Yuji is very shy and nervous about meeting girls and has a weak personality and no spirit. He always comes out of a losing battle against his sister Yuri. But one day, Yuji met a traveling salesman who gave Yuji a board game to take home for free, hoping it will boost his confindence and spirit dramatically.
- Yuri Kashima
Yuri (鹿島ゆり Kashiima Yuri) is Yuji's older sister. Although she constantly fights with Yuji over even the smallest things, she cares about him as any other sister would. However, she's very high-strung and strong-willed from her rigorous training and sports competitions at her high school. Yuri may be strong and fit, but is lacking figure.
- Aoi Kashima
Aoi (鹿島あおい Kashiima Aoi) is Yuji and Yuri's mother. Aoi is kind and gentle and cares deeply for Yuji and Yuri, no matter what they get themselves into. Aoi is a dedicated and domestic housewife taking care of the house and family. Aoi has a curvaceous figure and is well-mannered, but after giving birth to Yuji, her husband may have lost interest in Aoi. His fate is unknown, but Aoi is shown living with Yuji and Yuri alone.
Sōkan Yūgi is a game that carries a strange curse. The game itself has 2 dice and a pack of cards. On the gameboard are white and red spaces. The rules state whoever lands on a red space must draw a card. The card depicts a sexual act that has to be performed. If a player refuses to comply, the curse forces the player to perform. The game's goal was to "aim for the top".