Season 1

Episode 1 "Days Gone Bye"

Sheriff's Deputy Rick Grimes is walking around a gas station parking lot looking for fuel. He finds a young girl walking through the gas station who turns out to be a zombie. He shoots her in the head as she approaches him. The show then flashes back to when he is critically wounded in a vicious gunfight and slips into a coma. He later awakens in an abandoned hospital to find a world occupied by the walking dead. Grimes finds his way home, only to discover that his wife Lori and their young son Carl have disappeared. He is upset and delusional, but encounters a father and son, Morgan and Duane Jones, who have holed up in a neighbor's house on Rick's street, and learns that the CDC has set up a quarantine zone in Atlanta, Georgia. Rick departs for Atlanta in order to track down his family, leaving Morgan and Duane with some weapons and a police walkie-talkie, which he will broadcast periodically to at dawn to inform them of his location. Elsewhere, Lori has begun a romantic relationship with Rick's old partner and best friend, Shane Walsh (who has told her Rick is dead), and along with Carl and a small band of other survivors, is in hiding in a rural area outside Atlanta. Rick arrives in Atlanta and is set upon by an enormous horde of "walkers". He survives by slipping into a tank and sealing himself inside. An unknown person sends a transmission to the tank's radio, saying "Hey you, dumbass. Yeah, you in the tank. Cozy in there?"
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Episode 2 "Guts"

Rick escapes from the tank aided by Glenn, the voice he had heard over the radio. Rick and Glenn meet up with Glenn's friends, a scavenging team of survivors that is a subset of the larger group that includes Lori, Carl, and Shane. Glenn's friends inform him that Rick's shooting spree has brought their hideout in a department store to the attention of the walkers. Emotions run high in the group, particularly between a black man, T-Dog, and an unhinged white man, Merle Dixon, which leads Rick to handcuff Merle to a pipe on the store's roof. As the walkers attempt to break into the previously safe store, brief radio contact is made with Shane and Lori's group who, unaware of Rick's presence, decide they are unable to help. Rick and Glenn hatch an escape plan and execute it by covering themselves in a "terminated" walker's blood and guts (to mask their scent as they move amongst the walkers), and making their way to a box truck. Glenn draws the walkers' attention away from the store by racing around in a Dodge Challenger and sounding its alarm. The group escapes the city, along with Glenn, leaving Merle chained to the pipe on the roof after T-Dog trips over a bag of tools and accidentally drops the handcuff key down a drain.
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Episode 3 "Tell It to the Frogs"

Returning to the camp with the department store survivors, Rick has an emotional reunion with his wife and son. Soon afterwards however, despite encountering a walker near camp, Rick decides to go against Shane's advice and return to Atlanta to retrieve Merle Dixon and the bag Rick had dropped in the street when surrounded by zombies, which contained guns and a walkie-talkie he needs to warn Morgan against going to Atlanta. Rick is accompanied by Merle's upset and distrusting younger brother Daryl Dixon as well as Glenn and T-Dog. Lori warns Shane to stay away from her family now that Rick has returned. Rick's return is a surprise, since Shane had told her Rick had died in the hospital. Tensions run high in the camp between a woman, Carol, and her abusive husband, Ed. A fight breaks out when Ed hits Carol and threatens the other women, and Shane takes out his anger on Ed by nearly beating him to death. The rescue team makes it to Atlanta and, once they are on the department store roof, they discover a hacksaw, Merle's dismembered hand, and blood on his handcuffs, but he is nowhere to be found.

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Episode 4 "Vatos"

While still in search of Merle, the group tries to retrieve the bag of guns that Rick had dropped on his previous visit to Atlanta. However, the group is attacked by several living Latino men who also seek the weapons. The group manages to grab an injured attacker, but several other attackers escape in a vehicle, taking Glenn with them as a hostage. After interrogating their own hostage, Rick and the others learn the location of their attackers' hideout, and head there hoping to make a prisoner trade. They are rebuffed by the Latinos, who demand the bag of guns along with a prisoner exchange. The threat of bloodshed is avoided when an elderly woman (the grandmother of one of the Latinos) disrupts the confrontation. Grimes and his men realize that the "thug" image their hosts portray is just a protective front. The Latino "gang" is made up of former employees of a hidden nursing home within which many of its elderly inhabitants are hiding. Rick leaves them some of the guns and the men exchange prisoners and depart. Upon leaving, they discover their van is missing and conclude that Merle has stolen it. Back at camp, a large group of walkers ventures up the hill and takes the survivors by surprise. Several members are killed, including Andrea's younger sister Amy, and Carol's husband Ed. Rick and the rescuers return to camp just in time to kill the remaining walkers. 
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Episode 5 "Wildfire"

While the dead are being buried, Andrea remains protective of Amy's body until it reawakens as a walker, whereupon Andrea then apologizes and kills Amy again. Jim reveals that he was bitten by a walker in the fight, and the living members of the group decide to take him to the CDC, though there are signs of conflict between Shane and Rick, as Shane believes the trip to the CDC is a dead end. One family from the group splits away, deciding to return home to Birmingham, Alabama, and the remaining members depart for the CDC. On the way there, while making repairs to the RV, Jim decides he wants the group to leave him so he can be reunited with his dead family (either figuratively in the hereafter or, perhaps, literally as a walker). The group continues on, arriving at the CDC building, where a scientist, Edwin Jenner, who isolated himself while running tests on the walker virus, has just lost his only virus test sample and is considering suicide. When the group prepares to leave, thinking the building is empty, Rick notices the camera moving and begs for his group to be allowed in. The doors open as the group of survivors looks on, astonished.
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Episode 6 "TS-19"

In a flashback to the initial walker outbreak, Shane attempts to move Rick out of the hospital but is unable to do so because of soldiers executing the hospital staff and walkers breaking into the building. Thinking Rick is dead, all Shane can do is barricade Rick's door and flee the hospital. In the present, the group enjoys an ephemeral normality amid the comforts of hot showers and wine. A drunk Shane seeks to defend his past actions to Lori, telling her that he had been certain that Rick was dead. He tries to force himself on her sexually but she fights him off. The next morning, Dr. Jenner explains to the group, using a computer model called Test Subject 19, the process in the brain that takes place when a person is bitten and then partially revived as a walker. Subsequently the group discovers that a timer in the lab is counting down to the exhaustion of the building's power supply and that when it hits zero the building will "decontaminate" by self-destructing. Dr. Jenner locks them in, telling them that such an end is humane, and their deaths will be swift and painless. The group, however, believes they can survive, and attempts to break out. Rick convinces Jenner to release them; Jenner does so and tells him something inaudible before the group flees. One of the members, Jacqui, tells them she will stay no matter what, and die with Jenner. Andrea chooses the same fate, but Dale says if she stays then he will stay and eventually convinces her to leave with them. Carol gives Rick a grenade she found in his clothes when she was doing laundry, and they blow up one of the windows and narrowly escape the building before it explodes, and then proceed to drive off.