An unidentified person wearing prison garb brings in a gas can and a deer corpse, dragging it into the prison. The unidentified person then tosses rocks at a couple of walkers to obtain their attention. The walkers feast on the deer carcass that has been strategically spread out to lure the walkers towards the prison gate. The person then breaks open the prison gate, placing the deer's heart in front of it.
At the prison, Daniel, Alexis, Kiko, Fábio, and Gonçalo are clearing the yard. Meanwhile, Cassandra, Leandra and Cristina are seen talking on the exterior of the prison, next to them, Ruth, Sophia, Mariana and Cristiana are also talking, while Diana and her boyfriend are walking around the prison exterior.
As Daniel and the others clear the yard, Inês and Beth Greene escort Hershel outside. The group rejoices at Hershel's recovery. However, the warm moment is interrupted as a group of walkers suddenly flood the courtyard. Gunfire erupts as Daniel and the others race to join the fight. Cassandra, Leandra, Hershel and Beth lock themselves in a fenced-in entryway while Maggie, Cristina, Caramelo and Bruno flee inside the prison. Sophia, Ruth, Inês, Cristiana and Mariana, run to Cell Block C and lock themselves in the cells. While resealing the courtyard gate with her girlfriend, Diana, Fábio is bitten on the shoulder by a walker before the couple flees to safety inside the prison.
Back in Woodbury, Johanne asks Marcelino if The Governor is a good man. "I wasn't in the best of shape when he found me," Marcelino replies. "He should've just kept on going. Yeah. He's a good man." Johanne then joins her friends near the walls of Woodbury.
In the prison yard, Alexis reports that someone had cut the gate open. Then, the prison's alarm sounds, attracting more walkers to the outer fence. Kiko explains the back-up generators must be powering it — proving that someone else is messing with things from inside the prison — and follows Daniel inside to disable them.
Wandering the prison hallways, Diana offers to "do whatever's necessary" to help Fábio with his bite, but he insists on guiding her back to the cell block before he dies. Cristina, Bruno, Caramelo, and Maggie are also wandering when the stress of the situation causes Cristina to go into labor. Bruno leads them into the boiler room.
Back at the cell block, Daniel and the group separates into teams to look for the others and shut down the back-up generators. In the boiler room, Maggie assists Cristina in removing her pants and gets her ready to deliver the baby naturally. Caramelo stares at them in confusion and horror while Bruno stands guard. But as Cristina pushes through her contractions, she begins to hemorrhage.
Fábio and Diana reach the laundry room doors, but their path is blocked by two walkers. Out of ammo, Diana looks to turn back, but Fábio, knowing that he is already doomed, charges at them, pinning them against a wall, urging Diana to keep going. She reaches the doors, but looks back in horror and sadness, crying, as the walkers begin to devour Fábio.
Back in Woodbury, Johanne and The Governor share a parting drink in his apartment, where he reveals that his wife died in a car accident before the apocalypse. Johanne reveals that she never got to see her family, not knowing if they're alive or dead. She told him about her friends and how she misses Sophia and Paquete the most.
Daniel, Alexis, and Gonçalo reach the generator room, where they find Andrew — still alive and waiting to attack them, revealing that he lured the Walkers and caused the attack as his revenge. As Andrew and Daniel struggle, Alexis picks up Daniel's gun and shoots Andrew in the head, looking scared for killing a living person.
In the boiler room, Cristina grows faint as the hemorrhaging continues. She realizes she will not survive natural childbirth. "I'm not losing my baby," she tells Maggie, "You have to cut me open". Cristina asks them to say goodbye to Cassandra and Leandra for her. Maggie reluctantly slices Cristina's stomach open and scoops the baby out. The baby survives, but Cristina lays motionless, blood pouring from her open wound. Maggie starts to leave with the baby when Caramelo stops her. "We can't just leave her here," Caramelo says. "She'll turn." Caramelo offers to shoot her. Maggie leaves the room to give Caramelo some privacy. Moments later, Maggie hears a gunshot.
Daniel, Fábio, and Gonçalo move through the prison, taking out a pair of walkers, and discovering Fábio's mutilated remains. Daniel finds Diana's scarf and gun, and looks at them horrified. The trio moves outside, where they reunite with Cassandra, Leandra, Hershel and Beth. As Daniel plans to go back and search for the rest of the survivors, the cries of a baby ring out. Maggie, Bruno and Caramelo emerge with the newborn baby. Inês and the other girls who had locked themselves in the prison cells come out. Daniel informs the group about Diana and Fábio after Caramelo told them that Cristina didn't survived. Beth hugs Daniel, who was crying, thinking about Diana's death, Cristiana and Mariana console Inês who was horrified and sad for the death of her best friend. Leandra conforts Cassandra who was crying because of Cristina as the others look on in shock and sadness.
- First appearance of Ben.
- Last appearance of Cristina.
- Last appearance of Fábio.
- Last appearance of Andrew.
- The Cover of this episode features Diana and the prison, representing Diana's disappearence.
- The title of the episode, "Killer Within," refers to Andrew being the "killer" inside the prison. He released the walkers into the prison, which killed Fábio and prevented Cristina from having a stable C-Section. The title may also refer to Cristina's baby, as it is the main cause of her death, albeit indirectly.
- During the walker invasion, it is shown that Hershel can still defend himself with his one leg and crutches. He is seen knocking a walker down with one of his crutches.
- This was one of the most emotional and difficult episodes to shoot for the cast and crew.
- Six-week-old twin brothers have been cast to portray Ben.
- In this episode, Diana was originally supposed to die, but the producers decided against it.