It's a story about a nurse saving the world.

The land of seville was invaded by the demons, and the elves who had previously ruled the land were wiped out. The heroes of the other races were also killed by the Lord Abysm--The leader of the demons.
Desperate, people gave up, hid or died.
The demons occupied race after race easily.
Finally, the demons came to the Bigcat town - the last town where people live.

The nurse's mission is to guard the last wall and save the people behind it all by herself.