Hi, I just got Dead Rising and would like to know if you have to get on..

Well if you do every story mission in the 72 hours you will then unlock "overtime Mode". In the story after the 72 hours the helicpter crashes and frank is stuck in Willemette Mall (a cutscene shows all of this). Then Overtime Mode starts right after the cutscene ends. In the cutscene you find out Frank is infected and must find a "potion" to slow down his transformation by into a zombie by 20 years. Then once you play through Overtime Mode Frank must escape with that girl with the black hair. (can't remember her name, sorry) They then escape through a tunnel infested with zombies and also using the "potion" as a repellent to zombies. Once out the tunnel you must flee from a tank and who is trying to kill you. The tank soon catches up and knocks your truck over. Then you must fight a boss called Brock on top of the tank. While you fight him stay on the tank by all means, there are about 100 zombies around the tank. Once you beat Brock a cutscene will show of Brock falling into the pool of zombies below. Then he looks at the girl with the black hair who is sitting on top of the knocked over truck. Then Frank starts screaming. Then writing will appear saying "Frank escaped but soon after there wear records of the virus spreading to other cities around the US". Then they will make "Dead Rising 2" from that storyline. After you have done all that you then unlock unlimited mode where there are no time limits or missions (i think there are no misson). I hope I helped.