Late at night, an unknown person is shown feeding a rat to some of the fence-walkers. The scene then swaps to Tyreese and Karen in the library, overlooking their lives and then Tyreese begins playfully singing to her. They both then head back to their respective cells. Karen takes a detour to the showers to fill up a pitcher of water, where she hears movement, but finds nothing. After she exits, a zombified Patrick rises out of the shower, undetected. He exits the showers and follows her back to the cell block. He stops at her room, but is distracted by the coughing of another person and wanders into his cell and proceeds to bite out the man's throat, preventing him from calling for help.
At dawn, Patrick has finished devouring the man, and the latter soon awakens as a walker. Glenn and Maggie are in the guard tower on watch, and Glenn manages to take a photo of Maggie with the camera that he retrieved from the Big Spot. Rick and Carl head out to tend to the pigs and Carl asks if he can help with the fence-cleaners, but Rick shuts the idea down. Carl apologizes and reassures his father that he is trying to live normally and asks if he can have his gun back. Before Rick can answer, they hear gunshots. Mika and Lizzie run out of Cell Block D, calling for help. A horde of walkers has attacked from within and killed several people. Michonne, who is heading out on a supply run, is alerted to the chaos, but gets trapped in the prison gates with two fierce walkers. Just as the walkers manage to overpower her, Carl kills one of them; the other is kicked off by Michonne and then shot by Maggie. Michonne accidentally injures her leg on one of the snares, but is helped up by Maggie and Carl. In the cell block, the group clears out the walkers. Carol assists Ryan Samuels into a cell after he is bitten on the arm and prepares to amputate it, but finds that he has been bitten on the neck as well. After the threat temporarily clears out, Daryl and Rick comb the area, searching for anyone bitten to ensure that they won't re-animate, when Glenn gets attacked by a walker. Daryl is able to shoot it in the head, and realizes that it was Patrick. Rick is initially hesitant about putting the people down, coming out of the first cell shaken and in a daze. Another bloody-eyed walker, Charlie, emerges from a cell and is put down by Rick. They realize that since Charlie locked himself in his cell due to his sleepwalking, that there was no way that he could have been bitten.
Rick, Hershel, Dr. Subramanian, Daryl, and Bob gather around Charlie's body, noting that he has no bites or scratches. Subramanian recognizes the blood pouring out of Charlie's orifices as an indicator of a disease, and this prompts Rick to mention Violet, the deceased boar that he encountered a little while ago. Glenn suggests that it's a new walker disease, but Subramanian says that he saw cases like this pre-apocalypse. Subramanian explains that diseases like the one that killed Patrick were spread by feral pigs and birds in the old days and Bob notes that they all thrive in close quarters, like the cell block. They conclude that everyone in the cell block could indeed be infected by the flu.
Ryan realizes that he is dying, and asks Carol to take care of Lizzie and Mika for him as if they were her own. Carol agrees, but also tells him that he has to let them say goodbye to him -- before she kills him. As Lizzie and Mika reluctantly do so, Carol tells them what she has to do it, but Lizzie volunteers to do it instead. At the last minute she is unable to stab her father in the head and turns away crying as Carol takes back the knife and does it herself...
Carol, Hershel, Daryl, Sasha, and Glenn think of what to do with the people who might have been infected, including a suggestion to quarantine them in Cell Block A; Former Death row. They then hear coughing outside of the room. It's shown to be coming from Karen, who was walking by with Tyreese. She is told that she must be quarantined in the Tombs while they find out what the cause is. Sasha (as well as Karen herself) calms Tyreese, stating that it is necessary. As she leaves, Karen also mentions that David was coughing as well. Carl and Maggie are carrying Michonne back inside when Rick approaches them. He tells them what happened and assures Maggie that Hershel and Glenn seem to be fine. However, he says that they must stay away from him for now, due to him possibly being infected.
Beth is then seen dressing Michonne's wound when the latter says that she made a mistake of letting the two walkers get the better of her. She bitterly says that Maggie and Carl should have left her. Beth says that she is being foolish. People getting hurt is part of living with a group. She muses about those who died, wondering what you call a parent who's lost a child. Michonne begins to tear up, but composes herself immediately and Beth doesn't notice. Later, she asks Michonne to hold Judith while she cleans up the mess Judith made on her. Michonne looks at Judith with indifference, but then loses control and weeps while holding Judith close (hinting that something horrible might have happened in her distant past).
Carol goes to talk to Mika and Lizzie, as they stand by the fence. She turns to Lizzie and tells her that Ryan asked her to take care of them and says bluntly that Lizzie has become weak, and that in this world, she can't be. Lizzie begins crying about how they killed some one, and Carol realizes that she is referring to Nick, the walker that she named in the previous episode. Lizzie runs away and Mika explains that Lizzie is not weak, but "messed up".
Daryl is digging graves for those who had died when Rick comes by to assist. Daryl tells Rick that he's earned his time away from being in the position of a leader. Then, Maggie screams for them to follow her. It's shown that a massive group of walkers, possibly drawn by the gunfire from earlier, is converging at a portion of the fence that is about to give way. Despite the efforts of Glenn, Sasha and even a few others, there are just too many walkers. Sasha points out the carcasses of several dead rats near the walls, suggesting that someone has been luring the walkers to the fence, which explains why they are clustering together, instead of spreading out. Rick, getting an idea on what to do, tells Daryl to get one of the trucks. The two lead the walkers a few meters away and Rick pulls out one of the piglets; He then cuts its femoral artery and leaves it for the walkers to devour. He repeats this several times before the walkers are a safe distance away. Rick appears to be mourning as he kills each piglet, and seems to be significantly affected when the last piglet's blood splatters all over his face.
Meanwhile, Carol finds Lizzie and Mika near the fence looking at walkers once again. Carol assures her that she needs to be strong in letting her father go and not to run away from her fear of the walkers but to face them. She puts a flower in Lizzie's hair. Lizzie takes the knife from Carol's hand, and proudly puts it in her belt.
Carl finds his father about to burn up the pig pen. Rick explains that they either got the virus from the pigs or that we infected them. He then tells Carl that they need to stay away from Judith for a while to protect her from possibly getting infected. Carl then tells Rick about Carol teaching the kids how to use knives, but asks that Rick not confront her, as he believes she is right in doing so. Rick promises not to. He then pulls out his gun and holster and hands it back to Carl, as he now fully trusts that his son will use it for good. Rick lights several matches and burns the pigpen to the ground; then, Rick takes off his bloody shirt and throws that into the fire as well. Tyreese is seen going to Karen's isolated cell in the Tombs with some flowers in his arms, only to find her missing. He notices a trail of blood leading down the hallway; Following it, he arrives outside where he sees the burnt corpses of both Karen and David; Karen is recognizable by her bracelet.
- Greg (Alive and Zombified)
- Mr. Richards
- Patrick (Zombified)
- Charlie (Zombified)
- Ryan Samuels
- Nick (Zombified)
- 4 unnamed prison survivors
- First appearance of Jeanette.
- First appearance of Noris.
- First appearance of "Noris' Son".
- First appearance of "Sick Teenager".
- First (and last) appearance of Charlie. (Zombified)
- First (and last) appearance of Mr Richards. (Zombified)
- Last appearance of Karen. (Alive)
- Last appearance of David. (Alive)
- Last appearance of Ryan Samuels.
- Last appearance of Greg.
- Last appearance of Chloe.
- Last appearance of Owen.
- Last appearance of Patrick. (Zombified)
- Last appearance of Nick. (Zombified)
- Last appearance of the piglets.
- The ending scene in which Tyreese discovers Karen and David's bodies is an adaption of the scene in Issue 15 in which Hershel finds his daughters' decapitated corpses.
- It is revealed in "The Grove" that Lizzie was the unknown person feeding the walkers at the start of the episode.
- The song Tyreese sings to Karen is Frank Sinatra's "I've Got You Under My Skin".
- The song Beth sings to Judith is Tom Waits's "I Don't Wanna Grow Up".
- When Rick checks his revolver before holstering it, there are no primers in his cartridges.
- When Glenn wakes up in the prison you can see two chairs leaning against the walls behind him when he stands up. These are still present in the second shot, but when the camera moves back for the third shot they have mysteriously disappeared.