The winter has passed and the survivors find a prison. Rick breaks them into the grounds and then insists they move on inside. They spend the night in one cell block before venturing further on. The able bodied adults are surrounded by Walkers and Hershel gets bitten in the leg. Meanwhile Andrea is ill and is being looked after by the mysterious sword-wielding woman.
The prisoners in the kitchen have survived for ten months and Rick is reluctant to trust them. He agrees to clear a cell block for them but they struggle to adapt to fighting Walkers. Their leader is particularly bloodthirsty and so Rick kills him and leaves another to be killed. The remaining two are given the neighbouring cell block. Meanwhile Hershel lies unconscious and Carl slips out to the infirmary on his own. Carol practices for Lori’s birth by cutting a Walker open.
Andrea and Michonne follow a fallen helicopter and watch as men come in cars to collect the only survivor. Merle appears from behind them and they are taken to “Woodbury.” The community of seventy three hide behind their walls while the Governor keeps a close eye on them. He assures Andrea and Michonne they are free to leave but asks them to stay. He then heads out to find the soldiers who the helicopter pilot tells him are out waiting for him. The Governor and his men murder them and take their supplies.
At Woodbury the Governor manages to persuade Merle not to go in search of Darryl and Andrea to stay another day. Michonne remains deeply suspicious. At the prison Rick is directing operations and refuses to allow the remaining prisoners to join the main group. Hershel manages to go for a stroll on his crutches when suddenly the prison sirens go off and Walkers appear from within the prison. Everyone scatters with T and Carol rushing in one door while Maggie, Carl and Lori go in another. Each group is now inside Walker territory and have to make horrible choices. T dies allowing Carol to escape while Lori gives up her life to save the baby.
Rick heads into the prison in trauma and begins hacking Walkers to bits. Darryl and Maggie race out to find formula for the baby. At Woodbury Michonne retrieves her sword and slaughters some caged Walkers she finds. She is convinced that she won’t be allowed to leave but when Andrea goes with her the gates are opened for them. Andrea stays behind but is appalled to discover what the residents do for entertainment.
Merle leads an unfortunate team after Michonne. She kills two of them and the Merle finishes off the fourth when he disobeys orders. Merle and Michonne then come across Maggie and Glenn who are out searching for formula. Merle kidnaps them and drives them to Woodbury while Michonne locates the prison. At the prison Rick answers phone calls hoping that it will lead to a safe place. Darryl finds Carol. At Woodbury Andrea admits that she likes fighting Walkers and sleeps with the Governor.
Merle tortures Glenn while the Governor torments Maggie. Andrea joins Milton in performing a memory experiment on Mr Coleman, an elderly man about to die of cancer. Michonne tells Rick where Maggie and Glenn have been taken and leads a party to go rescue them.
Michonne heads for the Governor’s room while Rick and the others look for Maggie and Glenn. They are successful and although Oscar dies they escape over the wall. Michonne kills the Governor’s daughter and stabs him in the eye. Darryl is captured and he and Merle are held in front of the residents of Woodbury who are baying for blood. Meanwhile back at the prison a new group of survivors make their way in through the back of the building and Carl has to take charge of rescuing and containing them.
Rick and Maggie return to Woodbury and cause enough chaos to allow Darryl and Merle to escape. Rick refuses to take Merle back to the prison and Darryl leaves with him. They do take Michonne. Glenn is angry and bitter over what took place as Hershel tries to patch everyone up. Tyreese hopes that his people can take refuge at the prison but Rick sees a vision of Lori and reacts in front of everyone. At Woodbury the Governor stays indoors as Walkers break in and Andrea is forced to restore order.
Rick leaves the prison in search of Lori's ghost. Darryl and Merle save a Hispanic family from some Walkers but bicker about their past. The Governor asks Andrea to lead Woodbury while he recovers. Glenn tries to pick up the slack at the prison but Hershel can tell that he is driven by his rage and is being reckless. The Governor launches an attack on the prison including driving a truck load of Walkers in through the gates.
Andrea asks Milton to help her escape and go to the prison. When the Governor approves the move she heads over there. She gets a frosty reception and hears the truth from Michonne about why she came back to attack him. Carol suggests that she assassinate him. She returns to Woodbury but can’t do it. In the meantime Tyreese and his friends arrive at Woodbury.
Rick, Michonne and Carl head back to Rick's hometown to try and gather weapons. Around the high street someone has set up an elaborate Walker trap and written messages of doom on various walls. A masked man shoots at them but they overpower him and it turns out to be Morgan. Rick talks to him and worries that he has lost his mind. Carl heads off to find a picture of Lori and Michonne helps him avoid some Walkers.
Andrea sets up a meeting between Rick and the Governor. She waits outside along Hershel, Darryl, Milton and Martinez. Inside the Governor demands Michonne in exchange for leaving the prison alone. Back at the prison Merle wants to go kill the Governor while Maggie and Glenn get their relationship back on track.
The Governor loads up a truck full of weapons and prepares a torture chair for Michonne. Milton shows Andrea the full horror of it all and she flees Woodbury. Tyreese lets her go and he and his friends are led off to help gather Walkers. The Governor stalks Andrea to an abandoned factory where she escapes and seems to leave him surrounded by Walkers. He manages to get to her though and stop her from reaching the Prison.
Rick tells Hershel, Darryl and Merle of his plan to hand over Michonne to the Governor. Merle doesn't believe Rick will go through with it and takes matters into his own hands. Michonne questions him on the way and suggests they turn back. He agrees in part and lets her go. But he continues on to the rendezvous and kills a number of Woodbury men before the Governor gets to him. Back at the prison Glenn proposes to Maggie and Rick tells the others that they need to be a democracy again.
The Governor leaves a dying Milton in the torture room with Andrea. He then gathers his troops for war while our survivors apparently abandon the prison. Instead once inside the corridors Rick and co surprise them and drive them away. Out on the road the Woodbury army mutiny and the Governor guns them all down. Rick, Michonne and Darryl follow to finish things and find one survivor Karen. She helps arrange a truce with Tyreese and they all find Andrea has been bitten.