The story in Dead Rising: Case Zero acts as a prequel to the events seen in Dead Rising 2 and is set shortly after Chuck and Katey escap e the Las Vegas zombie outbreak, arriving in the town of Still Creek. Although it’s not necessary to play through in order to follow the main storyline, the plot gaps it fills in certainly helps to explain things a bit better before you’re thrown into a whole new city with a completely fresh protagonist.
Okay, so Case Zero may not be treated as a main entry in the franchise, but those that have played this lovely, compact DLC content will surely agree that it is worth a look.