The TF141 is doing everything it can to contain the undead threat. You and Ghost were tasked with carrying out an undead containment mission in Chicago. But unfortunately there were too many undead and Ghost sacrificed himself to give you time to escape while you watched as Ghost was overwhelmed by the undead.

Days after Ghost's death, you were given another containment mission, again in Chigago. But as soon as you return, and check the streets you find a Zombified Ghost wandering around