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.
It can't be too hard, can it?