Jessica McCarney is first met in the security room after Frank is the last survivor to stumble into the room. After Brad goes off back into the mall, Jessica and Frank exchange a few words. She sees Frank's camera and requests to take a look at his pictures. Frank notices her stopping at a picture of an elderly man. He asks whether he was important, but she does not reveal any information. After Frank leaves, she notices Brad being attacked by Carlito Keyes on the security monitors and leaves to aid him. While going through the warehouse she is attacked by Frank, who thought she was a zombie, and, while he did not hit her, the fall caused her to sprain her ankle. She has no choice but to give her handgun to Frank so he'll help Brad while she goes back to the security room.
The next day she spots Dr. Barnaby being taken by Carlito and has Brad and Frank go after him. When they return, she watches over an injured Brad while Frank goes to get him medicine. A few hours later, Jessie and Frank are talking about the girl from the supermarket and Dr. Barnaby wakes up. Jessie starts to interrogate him, closing the door on Frank for privacy.
Brad and Jessie are then waiting for Frank when he comes back with Isabela, who is badly injured. While questioning Isabela, she tells them about a research facility in Santa Cabeza where they were trying to create a way to create more cattle meat. Instead, they accidentally discovered the queen insects, which created the zombies. After the government discovered what happened there, they ordered a clean-up job on the place, wiping out almost every single resident. Dr. Barnaby then emerges from the closet, now turning into a zombie, and attacks Jessie. He is thrown off Jessie and, before sharing a few last words, is killed with a single gunshot to the forehead.
Jessie, Brad, and Frank then learn of Carlito's plan to blow up the mall, so Frank and Brad leave to stop him. Frank returns, successful in stopping the bombs, but Jessie is worried about Brad, who has not returned unaware of his grim fate. Isabela and Frank then leave to see if they can get information from Carlito's computer in his hideout. While they are gone, Jessie spots Carlito getting captured by Larry Chiang, owner of the butcher shop, and alerts Frank to go and investigate. After Larry is killed, Frank and Isabela able to free the lines, allowing Jessica to finally able to establish communication with DHS HQ, but then learns that DHS has denied any involvement and existence of such situation and that Special Forces are coming to kill all zombies and survivors to make sure no one knows what happened.
Two Special Force soldiers are able to open the once-welded Security Room door, and confront Jessie. They tell her that if she keeps quiet about the incident, she won't have to be taken into custody. Jessie says nothing, and just attacks the agent, revealing that she was bitten and infected by the now-deceased Dr. Barnaby. The soldier's partner, standing guard outside the office, hears the noise, and investigates. He walks in and sees his partner getting eaten by the zombified Jessie and suffers a familiar fate. If Frank reaches back to the security room, he finds the dead bodies and the zombie Jessie wandering nearby.