While zombie-Patrick is loose in the prison, he manages to make a meal of one of the residents before anyone is the wiser. So there are two zombies walking around by morning, and the day begins before anyone figures it out. But eventually panic sweeps the prison while a number of nameless residents go down, Rick has to kill his first walkers in some time, and Michonne's emergency reentry goes wrong, causing her to hurt herself and forcing Carl to fire his first shot since Rick confiscated his gun after the end of Season Three.
After the outbreak is contained and the cleanup work done (which includes Carol having to kill the pre-animated father of two of the girls, one of whom is that idiot walker-namer), Hershel and Dr. S. figure out that they may be facing an epidemic of pleurisy, a flu-on-steroids that can apparently kill in a day. So that's awesome.
While the gang figures out a quarantine situation, Karen comes down with the bug. Daryl tries to get Rick back into the captain's seat while burying the bodies, but soon they have a more immediate problem: concentrations of walkers at certain sections of the outer fence are starting to make it buckle. Rick reluctantly joins the fence crew, hoping it's not too late. Sasha discovers the dead rats indicating that someone's been feeding the things (and I think we know who), and Rick takes the reins at the moment of truth. He feeds the horde his entire squealing, bleeding piglet population -- which may or may not have been sick -- to draw them away from the fence while the crew shores it up, though it's clearly a bitter sacrifice.
Beth has to take care of both Michonne and baby Judith for a bit, and Michonne recoils from the infant as though it has flu-on-steroids. But then she ends up holding the baby close, so there's clearly some baby-related history there.
And Lizzie, the older of the two girls who just lost their father, breaks down when she sees that her favorite fence-walker has been killed. That chick's a mess, but after having a talk with her, Carol lets her have a knife anyway. That one's going to take some watching.
Carol asks Carl to continue to keep the secret of her "story time" sessions, which are of course spent training the kiddies on weapons. Instead, Carl reports it to Rick and advises him to let it continue, which Rick agrees to do. So look who's making decisions again whether he likes it or not. One of those decisions includes Rick returning Carl's gun and strapping his own back on. Well, his pig-farming days were over anyway.
Finally, Tyreese goes to visit Karen in quarantine, only to discover that she and a fellow sufferer have been killed, dragged out of the building, and immolated. It's a mystery who was behind it, but I think we can assume we're looking for a very committed germophobe.
It's the middle of the night with a full moon overhead, but the walkers at the outer fence aren't sleeping. And neither is someone else, who approaches from the inside of the fence. All we can see of this mystery figure are the two things he or she is holding: an illuminated flashlight (again with the plentiful batteries at the end of the world) and a live rat. The unseen figure holds the rat up to a gap in the chain-link fence, and a walker sucks its brains out. Well, I think we've just solved the mystery as to why the walkers have been concentrated in one area of the fence lately. But who would want them to build up enough to potentially push down that section? Well, not to give too much away, but we are likely to find out later that the rise of prison society has reestablished a rule that the zombie apocalypse should have killed: never attribute to malice that which can be easily explained by stupidity.
Also at night, Tyreese and Karen are canoodling in the prison library or maybe the warden's office, if the warden was a literate cuss. Karen senses that Tyreese isn't into it, and he confesses that he's thinking about losing Zach the day before. He soliloquizes about and how people who were once acquaintances are now as close as family. Then he gets self-conscious about talking too much and switches to singing, namely "I've Got You Under My Skin." Maybe a song with less macabre subject matter would be more appropriate under the circumstances. But after we get an oddly close look at Karen's distinctive rock paper scissors bracelet, and Tyreese makes a "your cell or mine" type overture, Karen demurs, saying if they start that they won't stop, and she's not ready. So for now they say goodnight.
Flashlight in hand, Karen makes her way through a prison corridor which seems free of walkers. She pauses to shake the last bit of juice out of the dying batteries. In the process, she happens to light up some kid art on a wall showing happy humans inside the fence and gray-hued walkers on the outside, one of the latter labeled "Nick." If you don't remember who Nick is, it'll come back to you later. Her stop is the bathroom, which of course is where we last saw the newly zombified Patrick at the end of last week's episode. He's fallen and he probably can get up, you know. She manages to survive the bathroom without having to face more than a couple of creepy noises and bullshit scares, then makes her way back to her own cell in D block, alone. I notice that in a homey touch, people's cell doorways have been hung with mismatched sheets for a minimum of privacy. Karen beds down on her bunk and falls asleep almost immediately, which is not to say that un-Patrick isn't up and about and following her after all. In fact, it follows her all the way to her doorway. But then it's distracted by the sound of a sleepy cough a few cells up the block and precedes that way instead. There, the Patrick-walker finds an older extra asleep on his bunk, and dispassionately chomps down on his neck. Arterial blood pumps out, but the victim never makes a sound, probably because un-Patrick started off with his larynx. Lucky for un-Patrick.