Zombie Mice Annihilation
Mice fears cats because the latter is bigger and eats them. However, that would change if the mice become zombies. Well, things like that, happened in this world. The cats would not go without a fight and that is the start of the cats and zombie mice war. You get the idea, you just need to annihilate the zombie mice. Each kill would earn you money and some mice would also drop some valuable things you can use. You can use the money to buy some upgrades for your cat. The game would be harder as you make some progress.