Alert! Alert! OMG-Zombies! The City of Redfield has been hit by an outbreak of exploding zombies! For cool and unimportant reasons these suckers literally explode when shot, showering their zombie brethren with gore and causing chain reactions of showering zombie gibs with just a single bullet!
For reasons that aren’t important right now, all but one of your team is killed before arriving at the outbreak’s epicentre, leaving YOU to deal with thousands of zombies ALONE.