A lengthy flashback shows The Filthy Woman, whose real name is Martha, was in a car accident with her husband. She attempted to flag down passing cars for help, but no one ever stopped.
Martha's husband dies from his injuries, and when he returns as a walker, Martha is forced to kill him again, and burying him on the side of the road. Days pass as she never leaves the gravesite, and rapidly loses her sanity.
One of Polar Bear's trucks arrive and drops a supply box off on the side of the road, but Martha ends up killing the woman trucker, claiming that people need to help themselves. A montage shows Marth killing truckers by using the previous victims as walkers.
Martha hears a someone speaking on the trucks radio and when thinks she's speaking to Polar Bear, we learn that Martha was the woman speaking to Morgan at the truck stop when he first arrived in Mississippi.
Following Martha's attack on the truck in present time, the group leaves the truck while Al attempts to confront Martha, but June intervenes and gets attacked by walker Quinn. Wendell gets a shot off on Martha that barely wounds her, while Morgan kills Quinn.
The damage the truck's engine sustained during the attack causes it to catch fire and ultimately leads to the truck exploding. With the supply boxes destroyed, Marth drives away in the van.
The sound of the explosion attracts nearby walkers eventually turns into a herd and the group is forced to flee.
Morgan leads the group to a nearby hospital as the herd grows. The walkers eventually make their way into the hospital and the group is forced to head to higher ground.
They decide that they need to make it to the roof but need the elevator. Morgan and Jim attempt to find another way out, but are attacked by walkers. Morgan eventually kills two walkers while Jim is able to kill one that was attacking him. The two make their way back to the group.
Al finds the generators and powers up the building just before the group is cornered and are able to escape into the elevator.
On the roof, June attempts to check on Jim and finds that he has been bitten.
June asks Morgan what the plan is, but Morgan doesn't know what to do. June is confident that he will think of something.
Alicia and Charlie find the remains of the truck, and while Charlie says they should search for them, Alicia says they could be anywhere. During their hike, Alicia reveals that they aren't looking for the others, but that Alicia is taking Charlie to the beach because she simply wants something nice to happen for once.
They eventually come across John's hat in a nearby lake and discover John and Strand across the water.