The main character has been reincarnated into another world where humans and demons continue to fight for a long time.
In order to end the war and restore peace, she is presented with a condition at the time of reincarnation that she will give him one special ability of her choice.

So what she came up with was that if the hero who came to subdue the demon king and the demon king who was the leader of the demons were put together, neither the humans nor the demons would get hurt, and everything would end well.
And the hero gets a "forced heat magic" that makes everyone lustful on the spot.

At the time of the final battle, the main character casts a spell at the timing when they are alone in the demon king's castle.
All she has to do is watch over them and it will have a happy ending.

Of course, it's not enough just to watch over, and the brunt of the two people's forced estrus turns to the main character...