Z for Zachariah revolves around the life of Ann Burden, a teenage girl who survives a nuclear attack but lives in isolation in the belief that she is the only survivor after the radio stations became silent. Ann is the daughter of American farmers living in a valley that is so deep that it survives the effects of the nuclear attack. However, she is unable to venture out of the valley for fear of getting exposed to the radioactive effects that have made all the surrounding regions uninhabitable. Her parents and several neighbors set off to search for any survivors but fail to return, leaving her all alone in the valley. Ultimately, she survives by raiding the local stores for her daily supplies. 

Ann first suspects that someone is approaching, judging by the smoke she sees from afar and interprets this as a sign of human activity. It is in the first chapter that we are introduced to the idea that there could be another survivor of the nuclear attack. The second and third chapters revolve around the approaching individual, who is still unknown to her. Ann fears that her little hideout will be discovered, which will endanger her if the person has ill intentions. Before the unknown individual reaches the village, she tries her best to hide the signs of life and habitation by covering up the vegetable patch and making the house look deserted and unattended.

As the silhouette of the individual approaching becomes visible, Ann’s tension builds. The person appears donning a helmet and wearing a green suit, which she eventually makes out. Upon discovering the valley, the man looks around carefully, puzzled that there are signs of life — all the while when Ann remains hidden in fear of coming out to meet the stranger. The following day, the man goes around and performs tests for radioactivity in the surrounding environment. After seeing how the results came back clean, he immediately jumps into one of the streams for a swim assuming that it is also safe. Ann fails to warn him that the stream is affected.

The following three chapters revolve around Ann’s eventual meeting with the man after he falls sick from his exposure during the swim in Burden Creek, which was polluted with the radioactive water. Her brother’s dog, Faro, also comes back to the village and ends up befriending the man. The man’s attempts to feed it were unsuccessful, and Faro eventually turns to Ann for food. In Chapter Six, we learn the name of the stranger as John Loomis, a chemist from Cornwell University. During his delirious state, he reveals how he came to have the safe-suit, and that he had murdered his colleague, Edward, who had tried to take it away from him.

From Chapters Seven to Ten, we get to see Ann and Mr. Loomis working together as a team. Ann helps Loomis recover by providing meals and tending the farm. They work alongside from time-to-time on the garden and making arrangements for the use of the tractor. Mr. Loomis helps her to create a gasoline pump and also fixes the tractor, which she uses to plow the land in preparation for the planting of crops. At the same time, Mr. Loomis continues to conduct radioactivity tests on the immediate surroundings, as well as keeping a check on whether he was exposed after that fateful swim. Over time, his condition gets notably worse, where he experiences periodic episodes of delirium.

In Chapters Eleven and Twelve, Mr. Loomis’s condition deteriorates and he continues to be delirious. From time-to-time, he is drawn back to the altercation with Edward. It is at this point that we get to learn more about the fallout, which allows Ann the opportunity to connect the dots pertaining to the murder. Despite the disturbing discovery that Mr. Loomis is a murderer, Ann still goes on to pray for him in the chapel. She finds herself in a dilemma, torn between trusting the only person that she has come in contact with, or getting away from him and staying in her own corner of the valley. Nevertheless, she carries on in helping Mr. Loomis with his gradual recovery, which is demonstrated by his improved breathing.

In the subsequent chapter, Ann is in a more reflective state. She reminisces about her desire to be a school teacher before the war. At the same time, she contemplates stealing the safe suit and leaving for Ogdentown to grab some books from the library. Mr. Loomis finally wakes up from his recovery bed but is still too weak to fend for himself. Ann later celebrates her sixteenth birthday by baking a cake.

As Mr. Loomis recovers, he begins to take greater control of the farm — and Ann. He becomes overly possessive of the farm, which causes a strain in their relationship. Mr. Loomis fully recovers by Chapter Fifteen and where Ann observes a change in his personality, specifically where he becomes overly critical of what she does and of her ideas. For instance, when she proposes to personally drive into town and visit the library, he is quick to discourage her and suggests the reading of technical books instead. He also cautions her against taking the safe-suit. All in all, Mr. Loomis attempts to take control of the valley and worries more about his survival, reminding Ann that they should instead view it as the beginning of a colony and treating it as the whole world. His apparent shift to becoming an increasingly-selfish individual starts to make Ann very uneasy. 

One day, Mr. Loomis reveals his dark side when he physically accosts Ann when she prods him to talk about his family:

“And instead of letting go he tightened his grip, pulling me further off balance. I could not help what happened next. I felt myself falling from the chair, falling towards him, and quite instinctively I threw my right hand up (he was holding the left one) to catch myself. It hit him in the face, not very hard, on or near his left eye. At that moment he pulled back and relaxed his grip. I snatched my hand away and sprang back…” (O’Brien, 1975).

In reflex, she hits him and quickly apologizes, to which Mr. Loomis reacts by reaching for his cane and heading to bed. Once the food is ready, they eat in silence. In Chapter Sixteen, Mr. Loomis makes another attempt on Ann, where he slips into her bed while she is asleep, and starts caressing her. She hits him with her elbow and jumps away, narrowly escaping an assault. The cave was the only safe place for her away from Loomis’s advances, she was the only one who knew where it was, that coupled with the sense of security and freedom she considered it a refuge.

In Chapter Nineteen, Ann makes herself comfortable by getting a new pair of shoes and a change of clothes from Mr. Klein’s store. She then proceeds back to her cave, where she fears that Faro will spoor her out. But despite Ann’s hatred for Mr. Loomis, she still has a soft spot for him and leaves some supplies on the porch so that he doesn't starve. Eventually, they both reach an agreement whereby she would leave him the house, on the condition that she can live in peace on her side of the valley.

The next two chapters delve into the dynamics of the relationship between the only two residents of the valley. Things become so strained that, one day, Loomis even chases Ann down and shoots at her. She flashes back to when the valley was all hers, and to the alluring peace, she had felt back then. Throughout Chapter Twenty-Two, Mr. Loomis attempts to track Ann with little success, and eventually resorts to further mistreatment by cutting off her supplies, with the hope that it will lure her back to him. Finally, in the final two chapters of the book, Ann plots to steal the safe-suit and find a place far away from Mr. Loomis where she will feel more comfortable. With the help of Faro, he attempts to track her down; but she cleverly leads them on the wrong path. Faro ends up in a polluted pool of water and dies of radioactive poisoning. Ann finally gets a hold of the safe-suit after tricking Mr. Loomis out of the house. In the end, she puts on the safe-suit and sets off leaving Mr. Loomis, distraught and helpless. However, he extends an olive branch by pointing her to the west, where he says he had spotted some life on his way to the valley.

Setting

The events of the story are set predominantly in Burden Valley, which is a remote area in the U.S. Because of its self-contained weather system, it has allowed some life to continue even after the nuclear attack wiped out signs of life elsewhere. The protagonist’s forefathers gave the valley its name after settling there and building the farm, which extends for around four miles as it borders the road that enters and leaves the valley. We get a description of the landscape and the valley in its entirety from Ann’s recount as she sits on her vantage point at the entrance of her hideout, the cave.

“I am sitting at the entrance to the cave. From here I can see most of the valley, my own house and barn, the roof of the store, the little steeple on the old church (some of the boards are off the side—can I fix them? I don't know), and part of the brook that runs by about fifty feet away. And I can see the road where it comes over Burden Hill, and almost to where it disappears again through the pass—about four miles altogether…” (O’Brien, 1975).

The valley is predominantly woodland, and within it is Burden Creek. It is populated by numerous animals, including two cows and a bull calf, a chicken, several rabbits and squirrels, a few crows, and Faro the dog. There is also a stream that passes through the valley on the eastern part of the hillside, feeding the lake with fish that were a source of food for Ann, and continues on further before ending up in Burden Creek. Any land beyond the valley now lies desolate, where the only known life that remains is within the valley — yet, in the concluding chapter, this established fact is challenged upon Mr. Loomis’s revealing of possible life towards the west, just as Ann prepares to venture out beyond, wearing the safe-suit.