As the Oni horde grows stronger and stronger, it becomes clear that defending against their might is fruitless. Many of the powerful kingdoms have fallen, and the heir to a small merchant kingdom with hardly any soldiers to boast decides the only way to save his people is marrying one of the two most powerful bachelorettes in the known world - Princess Mai and Princess Tetsuko.
The Oni court approves, but with a small caveat - if the marriage is to go through, he'll have to rein in the bickering pair's worst excesses, as well as teach them about ruling properly in times of peace.