After a herd of walkers overran the farm of Hershel Greene (Scott Wilson) at the end of the last season, Rick Grimes (Andrew Lincoln) and his group are now in search of a new home. Several months have passed, including winter, and Rick's wife Lori (Sarah Wayne Callies) is due to give birth any day. After being chased out of an abandoned house by a large number of walkers, Rick and Daryl Dixon(Norman Reedus) go hunting for food and stumble upon a prison complex. With the help of the other survivors, Rick manages to enter the prison courtyard, lock themselves inside the first gate, and kill all of the remaining walkers. Noting that the walkers they encountered were all prison inmates and guards, Rick believes the prison fell early and thus could still hold a large supply cache. As the group settles in for the night, he urges them to continue onward into the prison in the morning.
Meanwhile, Andrea (Laurie Holden) has fallen severely ill since being separated from the group and being taken in by Michonne (Danai Gurira). Michonne has kept Andrea in a meat locker and encounters walkers while raiding a pharmacy to retrieve aspirin to reduce Andrea's fever. Andrea feels like a burden to Michonne and tells her to go without her, but Michonne refuses and sets out for a safer refuge with her.
The following day, Rick and the others manage to clear the prison yard, circling up and moving as a unit to clear all walkers blocking their path. Daryl notes that one of the walkers they killed was a civilian, hinting at a possible breach of the prison's security. The group clears out a cell block to sleep in. Lori confides in Hershel, believing that her relationship with Rick and their son Carl is ruined. She thinks she has miscarried the baby after feeling no movement, and worries that the baby might reanimate inside her and devour her from the inside out, or that she could die during childbirth and endanger her family. Hershel calms her down and tries to get her to not worry about it. Rick, Daryl, Hershel, Glenn (Steven Yeun), Theodore "T-Dog" Douglas (IronE Singleton), and Maggie Greene (Lauren Cohan) go on a scouting mission to other parts of the prison. They forage through the pitch darkness until further progress becomes impossible. They get split up after stumbling upon a cluster of walkers and Hershel, in an attempt to back up and locate the missing Glenn and Maggie, gets bitten on the leg by a walker. Rick chops off the bitten limb with a hatchet, in hopes that amputating the affected body part can stave off infection. As Hershel passes out from the pain and blood loss and Rick scrambles to find something to stop the bleeding, Daryl discovers that they are not alone – there are other survivors in the prison.