In the future, it possible to get back in time with the help of time machine. The future is not so great since it is being ruled by the undead. Somehow, you need to change that by traveling back in time. Only then you can save your own time line. The idea here is to get rid of the zombies. You would need to travel back in time to be able to do that. Each kill would earn you money. You can use the money to buy some upgrades for your character. The game would be harder as you make some progress, so you would need those upgrades.