The group has been on the run moving from house to house through rural Georgia for the last eight months since the end of Season 2. Carl has now become more proficient with firearms. Daryl seems to have officially replaced Shane Walsh as Rick's right hand man and Lori's pregnancy is near term.
Moving as a cohesive unit, Rick, T-Dog, Daryl, and Carl breach a house, eliminate the walkers inside, and search the place to make sure it's safe for the group to camp out there. After the search is over, the others enter the house. After a short while, everyone starts to settle until T-Dog looks out of the window and notices a group of walkers approaching. Everyone runs to the cars and they drive off.
The cars stop on a highway where Beth keeps watch whilst the group discuss where they are headed. They open a map and discuss how they have been moving in circles for months, trying to escape the herd that passed on the Greene Family Farm. T-Dog and a few others head to gather some water, while Daryl and Rick go hunting. While hunting, they walk down a railroad, and observe a prison full of walkers. They've found shelter.
Rick starts to cut the fence so the group can get within the prison exterior. Glenn and Daryl then tie back the fence so no walkers can enter behind them. They start to run, stopping at the prison gates. Rick says someone has to get past the prison yard full of walkers to close the gate on the other side, Glenn offers, but Rick says he will do it. Rick tells Glenn, T-Dog, Beth, and Maggie to distract and kill the walkers through the fence. He tells Carol, Daryl, Hershel, and Carl to shoot walkers from the guard towers. Rick straps a rifle to his back and is handed some chains, Lori mans the fence. Rick heads towards the gate. He kills a few walkers on his way there and then secures the gates. He then enters a tower and with help from the rest of the group, shoots the remaining walkers in the yard to secure it.
Having taken the yard, the group sit down for a fireside meal and, at Hershel's request, Beth and Maggie sing an old Irish drinking song. Daryl, while keeping watch at the front gate, starts to give Carol a massage, as she stated her shoulder hurt from the assault rifle's recoil. She starts to jokingly flirt with him, and then they head back to the fire. After patrolling the fence, Rick makes plans to take the prison entirely, convincing the group that it'd be safer on the inside. He tells them how there will be food, weapons, and medicine in the prison, and that "these [walkers] assholes don't stand a chance".
The next day, Rick, Daryl, T-Dog, Glenn, and Maggie take the interior while the rest serve as a distraction. Everything seems to be going well until they run into walkers wearing riot gear. However, they're taken out and the group advances into the prison. They enter Cell Block C, take out the few remaining walkers lurking inside, and find it to be secure. As everyone starts to set up in different cells, Carl starts to show an interest in Beth, then goes off to find his own cell.
Meanwhile, Michonne and Andrea aren't faring well. They've taken refuge in a deer cooler. Andrea is sick with influenza and Michonne makes Andrea take some aspirin she recovered from a store. Andrea presses Michonne to abandon her, but she refuses. They conclude they must move on as a group of walkers are advancing and Andrea will die if they stay, so they head off out towards the woods.
Lori confides in Hershel that she fears the baby might be stillborn, and that it might be born a walker. She fears that Rick and Carl hate her, and that she sometimes wishes she had died on the farm. Hershel assures that it isn't true and that the baby will love Lori. Nonetheless she makes Hershel promise that if the baby is a stillborn or if she and the baby both die in childbirth not to hesitate, and put them both down. The group finds a plethora of weapons and suit up to further explore the prison. Rick, Daryl, T-Dog, Glenn, Maggie, and Hershel move deeper into the interior while Carl stays behind to guard the women. All seems quiet at first, but suddenly they run into several groups of walkers. In the darkness and confusion, Glenn and Maggie get separated from the group. When it's finally safe to venture back out, Hershel tries to find the two but is bitten on his calf by a walker that only appeared to be dead. The group then reunites and assists Hershel in getting out of the corridor. They take refuge in the cafeteria, where Rick, in an attempt to save Hershel, amputates his leg, causing him to go into shock and pass out from the ensuing blood loss. Then, five shadow figures appear in the adjacent room. Daryl prepares to fire, but realizes they are not walkers but are actually more survivors living in the prison. He shines his flashlight in one of the prisoner's faces, to which Axel, one of the prisoners exclaims, "Holy shit!"
- First appearance of Axel.
- First appearance of Tomás.
- First appearance of Oscar.
- First appearance of Andrew.
- First appearance of Big Tiny.
- This is the first episode where Lauren Cohan (Maggie) and Scott Wilson (Hershel) are listed among the main cast, after guest starring in previous seasons.
- Danai Gurira (Michonne) makes her first official appearance in the show in this episode. She is listed as a series regular, but unlike McBride and Wilson, she is included in the opening sequence.
- The song Beth sings in this episode is "The Parting Glass", a Scottish traditional song, often sung at the end of a gathering of friends.
- This episode has the most walker kills to date with a total of 59 kills (56 confirmed).
- This episode is the first season premiere that does not feature a human death.
- Like Allen & Dale in the Comic Series, the leg of Hershel Greene was amputated after being bitten while stepping over a walker.
- In the Comic Series, the original intention was for Hershel to lose his leg instead of Allen.
- The name of the episode, "Seed", may refer to when Hershel suggests that they plant seeds to grow tomatoes, cucumbers, and soy beans within the prison yards. It may also refer to the idea that they have begun planting their own 'seed' within The Prison, which will eventually provide a new life for them, or it may even be referring to Lori's unborn baby.
- During the 2013 NYC Comic-Con for Season 3, Andrew Lincoln stated that the number of walker kills in this episode is more than they had back in the first season.
- When Carl is rifling through the kitchen in the farmhouse in the opening scene, there is a yellow can of "SMEAT" on the counter... the fictional SPAM substitute made famous by Dennis Hopper's character in the movie Waterworld.
- Rick's group is entering the prison yard and killing the walkers with various weapons. However, none of the slides on their pistols move when rounds are fired, and no spent shells are ejected. Also, the guns seem to have little to no recoil at all
- After the group overtake the prison field, Glenn goes to kill a walker that wasn't killed and Hershel looks back to watch Glenn kill the walker. In the next cut, Hershel looks back again and Glenn had already killed the walker.
- The grass in the Prison yard is shorter than what it should be, considering there's nobody around to mow the grass.
- When Hershel is firing the M4A1, the aimpoint scope is mounted backwards.
- When Rick closes the gate to stop the walkers from getting in, it zooms out and the walkers have disappeared.
- When Daryl points to the walker and says there are civilians here, You can see the dead walker blinking.
- When the group enter the cafeteria for the final scene, there's a flashlight already in there, on the floor with its light on.
- When Daryl, Maggie, T-Dog, Glenn and Rick are killing walkers with mêlée weapons on their way inside the prison, there is a random CGI blood spatter that appears on the crossbar of the fence in the foreground.