We pick up before where we left off, but from the walkers' perspective. We see how this massive herd was in the city, saw a helicopter and decided to follow it, which led them along the highway and into the woods. They might have bypassed the farm entirely, or had gotten trapped in the swamps, but then heard the gunshots from the Shane/Rick/Carl incident and opted to head towards that. See, Carl ruins everything.
Glennn and Daryl return to the house and give a brief rundown on the whole it-seems-like-Shane-maybe-just-up-and-killed-Randall-but-somehow-the-dude-still-turned-into-a-walker thing. Everyone is all freaked out and Lori begs Daryl to go find Rick and Shane who still haven't returned.
Out in the field on their slow, leisurely stroll back to the farmhouse, Carl starts asking questions about exactly why Shane ended up dead and a walker and Rick isn't quite willing to answer them. How to explain to a child that his sometimes father figure tried to kill his actual father? Anyway, even though there are thousands of walkers and they've been coming up rather steadily and Rick's supposed to be a cop and have decent senses of perception, it seems to take him awhile to finally notice the hissing/plodding noises that the undead are making. He panics and then they run for it.
Back in the farmhouse, mother of the year Lori finally realizes that her son is missing, despite the fact that she told him to stay upstairs. Because telling him to stay some place has worked just so well in the past. Also, he's got that ginormous ugly ass hat on all the damned time, so you'd think someone would notice his movements. Lori wants to run after her son, but with all the zombies descending on the place, that's not really an option. She's freaked, and Carol gives her this look that is a combination of sympathetic and condescending. I'm almost starting to tolerate Carol.
Hershel wants to defend the farm, with the plan being to shoot as many walkers as they can, and use their cars to distract them and drive the herd away. It's not a terrible plan, and it's better than the previous plan to just hide out in the basement. The basement option is quickly dismissed after they realize that this amount of walkers would just tear the house down trying to get to them.
Rick and Carl make it only as a far as the barn. And they try and lock out the walkers, but this is a particularly feisty herd that will not be stopped for long by a door. So Rick sends Carl up to the loft, while he throws some gasoline all over the hay. He gives Carl a lighter and tells him to drop it when he tells him too. Then Rick lets the walkers in, starts climbing the ladder and screams to Carl to drop the lighter. It seems like he fumbles with it, and when he drops it, for a moment I really thought it was going close itself during the fall, failing to ignite. Maybe I've just come to expect ineptitude from the kid. But it seems like the show doesn't want Carl to die just yet, so they let the lighter thing work and the barn goes up in flames. Actually, given the fact that he has Lori and Rick for parents, he probably plays with lighters all the damned time without anyone noticing.