Your mission is to survive as many shadow attack waves as you can.
Each shadow you kill gives you cash; use cash to buy weapons. You also have a flashlight, when you shine it at your shadow enemies, they cower away from you! Remember to use your knife though, it's vital to your survival!
Watch out, every 10 waves there's a boss, and that's no walk in the park. For how long do you think you can survive?