You are a lone survivor and the zombies keep coming. Your gun fires automatically at the nearest zombie, so you only have to move and stay alive.
Controls: drag your finger anywhere on the screen like a joystick, or use W A S D or the arrow keys. Defeated zombies drop blue gems. Collect them to level up, then pick one of three upgrades (faster fire, more damage, extra bullets, speed and more). Some zombies are fast, and big brutes show up later. Survive as long as you can.