In the middle of the night, an unidentified person feeds a rat to walkers at the prison fence.
Meanwhile, Tyreese and Karen kiss in the library. Tyreese muses on the sadness of losing friends when there are so few people left in the world.
Afterwards, a fully reanimated Patrick follows Karen to D block. He wanders into a cell and begins silently eviscerating its sleeping occupant.
The next morning, Michonne prepares to resume her hunt for The Governor. Nearby, Carl asks Rick if he can help kill walkers at the fence but Rick prohibits it, refusing to say when Carl can have his gun back.
Gunfire erupts inside the prison, causing Michonne to turn around on her horse just outside the perimeter. "Walkers in D!" Glenn yells. Daryl confirms there's been no breach.
Carl lets Michonne back in through the gate. As she dismounts, walkers tackle her to the ground. Carl and Maggie rush to her aid, shooting the walkers as Michonne throws them off. Michonne sprains her ankle in the process.
Inside D block, Rick and his crew evacuate the survivors and kill the attacking walkers. Carol brings Ryan, a wounded victim, to his cell and sees that his arm and neck are bitten.
Rick, Glenn and Daryl sweep D block for stray walkers. Patrick jumps out and attacks Glenn, but Daryl quickly fires a crossbow bolt into his head.
On his deathbed, Ryan asks Carol to look out for his daughters, Lizzie and Mika. Carol agrees.
Rick, Hershel, Daryl and Bob examine a dead walker, Charlie, in his cell. "No bites, no wounds, I think it just died," Rick says. They deduce that Patrick and Charlie contracted a virus that was probably spread by the pigs Rick has been tending. Everyone in the prison might have been exposed.
Meanwhile, Carol brings Lizzie and Mika to say good-bye to their father. After he dies, Carol hands Lizzie a knife to stab him before he turns. Lizzie takes the knife but loses her resolve and suffers a panic attack. Carol quietly finishes the job as Mika comforts Lizzie.
Out in the courtyard, Rick debriefs Maggie, Michonne and Carl and warns them to keep their distance from anyone who was in D block.
The Council meets in the library and decides to quarantine those who were exposed. They walk out to find Tyreese escorting an unwell Karen to her cell. They explain the outbreak and insist on quarantining Karen and another survivor, David, who's also showing signs of infection.
Carol finds Lizzie and Mika at the perimeter fence. Carol gently scolds Lizzie for failing to kill her father's corpse. "You're weak," she says. "You have to act fast every time."
"He's dead," Lizzie cries. "He was special and now he's dead."
Carol assumes Lizzie is referring to her dad, but then Lizzie clarifies: The walker at the fence with the "Nick" name tag was culled.
"She's messed up, she's not weak," Mika tells Carol after Lizzie runs away.
As Daryl and Rick dig graves, Rick admits that he could have been more useful in the fight if he'd had his gun. Daryl says Rick earned his time away from the violence - they wouldn't be there if it weren't for Rick.
Maggie calls Rick and Daryl over to the perimeter, where a giant, clamoring mass of walkers is causing the fence to buckle. They start slaying the walkers to try and prevent it from collapsing completely.
Meanwhile, Beth wraps Michonne's leg and notes that there are names for widows and orphans but not for parents who've lost a child. Michonne considers this as Beth takes a crying Judith out of the cell.
Outside, the fence continues to cave. Sasha finds dead rats and realizes someone has been feeding the walkers. Rick hatches a plan and orders Daryl to fetch the truck.
Inside, Carol asks Carl not to tell Rick about her weapons lessons with the kids, worried that parents would object if they found out. Carl says he won't lie to his dad. "I'm not asking you to lie," Carol says. "I'm asking you not to say anything."
Meanwhile, Beth asks Michonne to hold Judith for a moment. Michonne holds Judith reluctantly at first, then tenderly. She tears up as she cradles the baby.
Outside the fence, Daryl drives a truck with Rick and the pigs riding behind on an attached flatbed. One by one, Rick slices the pigs' legs and sends them hobbling off, luring the walkers away from the fence with their squeals and blood. Glenn and Sasha, meanwhile, shore up the fence with wooden braces.
Carol finds Lizzie and Mika watching walkers at the fence. Carol insists Lizzie's sadness is about her dad, not a walker. Lizzie agrees and quietly breaks down. Then, Lizzie quietly reaches for Carol's knife and slips it in her belt.
As Rick dismantles the pig pen, Carl tells him that Carol has been teaching the kids to use weapons. Rick thanks him for being honest and agrees not to stop Carol. He lights the pen on fire then gives Carl his gun back. He retrieves his own gun and holsters it.
Tyreese visits Karen's cell with flowers but finds it empty. The bed is soaked with blood. He follows the trail of blood outside, where he finds two charred bodies on the ground: Karen and David have been murdered.