Toushin Tournament -- an annual event held in the Toushin City. All contestants are required to register with a beautiful female partner, and the winner of each battle gains the privilege of doing anything they want with the loser's partner (except harming them). The final winner of the tournament earns the title of Toushin, and gets to live freely in the Toushin District of the city with his partner.

Five years ago, Nacht's father participated in the Toushin Tournament, saying he'll be back when he wins. That was the last Nacht ever heard of him. Now, having grown up and hoping to discover the fate of his father, Nacht sets out to enter the tournament his father disappeared in.

[Taken from the AliceSoft Wikia]