Its been a year since the outbreak, but upto to these days, there seems to have no cure for the zombie infection. Once you turned into one of them then that will be your fate. Each day, the survivor are getting fewer because the zombies are eating them up. The government know that they have to rescue the survivors or there won't be anyone left. The government hired you because you have the experience and the guns to do the job. You know that you need to do this quickly or there won't be much to rescue. Besides, you are paid to do it and money is the greatest motivator of them all.
Drive around with your truck and pick up required number of survivors to be able to proceed into the next level. you can equip your truck with a bumper that you can use to fight the zombies. Once, the bumper is equipped, you can ram over the zombies without getting your truck damage. If your truck takes too much damage then the game is over. Try to pick up the survivors and get back to the safety zone as fast as you can. The game gets harder and harder as you make some progress.