The story was about a system reviving dead people to fight monsters. It's like a game the revived kill the monsters to earns points hoping to earn enough points to get freedom, they get killed they die again and for good bluh bluh...
But no proper explanation how/why this system exists. You won't probably understand it if you haven't read the manga or watched the anime just like me.
So I would suggest don't bother watching it unless you are also interested in related anime/manga.
