Age of Iron is narrated as a letter from Mrs. Curren, the protagonist, to her daughter, who remains unnamed throughout the novel. Curren lives, and has lived her entire life, in Cape Town. The novel takes place in the late 1980s during states of emergency throughout South Africa due to resistance and demonstrations against apartheid policies. At the start of the novel, Curren has just been told by her doctor, Dr. Syfret, that her cancer is aggressive and advanced, and she realizes that she likely doesn't have long left to live. Upon arriving home from the doctor's, she turns away a homeless man who is camped on her property. He leaves, but he doesn't stay away for very long. Mrs. Curren gives him food and offers him work, and when he rejects the offer for work, she lectures him about wasting his life. He spits at her feet. Later that day, she spots him staring at the TV through her window. Curren is unsettled by his presence. Later that night, however, she has a sudden painful attack, and the man helps her. They slowly form an unlikely friendship as the man, named Vercueil, spends most of his time near her house. One day she asks him to mail a package to her daughter, but only after she dies. The package will contain a collection of letters to Curren's daughter, among them the contents of Age of Iron. Vercueil hesitates, but after some convincing, agrees to mail it.
Mrs. Curren's housekeeper, Florence, returns from a trip and brings her two young daughters, Hope and Beauty, and her fifteen-year-old son Bheki with her. Mrs. Curren resents having Bheki in the house, but he has no other place go because of the violence in Guguletu. After a few days, one of Bheki's friends, who goes by John, starts hanging around the house. Curren especially doesn't like John; she finds him stolid and uncharming. Curren is also upset by the fact that John starts sleeping in her garage without her permission. Bheki and John beat up Vercueil because they don't like the way he lays around and drinks, so Vercueil disappears for a few days. Around this time, police officers start parking near Curren's house, apparently keeping tabs on Bheki and his friend. Tensions continue to rise. When Vercueil returns, he brings home a woman and they both pass out drunk in the living room. Overwhelmed by all of the people, Mrs. Curren starts to feel like everyone is conspiring against her to take over her property before she even dies.
One afternoon, Mrs. Curren witnesses the same cops who have been parked by her house chase Bheki and his friend through the street. The cops are in a van, and the boys are on a bike. They lose control of the bike and slam into a utility truck. John suffers a bleeding gash on his forehead and loses consciousness. Florence ushers Bheki inside, leaving Curren to tend to John until the ambulance arrives. Later that night, Curren drives Florence and Bheki to Woodstock Hospital. She insists that Vercueil accompany them in case the car fails to start. They search through Woodstock, but John is not there, so they try Groote Schuur. At the second hospital, Vercueil and Curren wait in the car because Curren is in too much pain. She also feels like the other elderly people in the hospital, the patients, can tell that she belongs among them. In the car, she tells Vercueil that she hasn't told her daughter the extent of her illness. He encourages her to tell the truth, so her daughter doesn't resent her after she's gone. At home that night, Mrs. Curren invites Vercueil to sleep on the couch.
Tragedy strikes again when Florence gets a phone call in the middle of the night saying her son is in trouble. Mrs. Curren drives Florence and her daughters to Gugulethu, and then to a nearby settlement called Site C, where they meet Mr. Thabane, Florence's cousin. They drive to a part of town in chaos. There are fires and dead bodies in the street. People are screaming and protesting against white Afrikaner militiamen while police stand by, doing nothing but further tormenting the Black residents. Faced with so much destruction and fear, Mrs. Curren essentially throws a fit and is shamed for her privileged sensibility by Mr. Thabane. Eventually, they find Bheki. He and four other young Black men have been murdered. Their bodies are lined up on the floor of an abandoned shop, and their eyes and mouths are full of sand. Curren is horrified. Thabane questions her about how she feels about what she's witnessing here. He tries to make her see that she cannot simply turn her back on the crimes being committed here, simply because she doesn't have the stomach for it. Eventually, though, she finds her way back to her car and drives home.
After these events, Mrs. Curren shares with Vercueil her suicidal ideations. She's thinking about lighting herself and her car on fire and driving into a Parliamentary building, or as close as she can get to one. She is dismayed by how excited Vercueil seems by this idea. He continually asks Mrs. Curren when she intends to kill herself, but she keeps putting it off. She's conflicted because she loves life, but she also knows that a painful death lies ahead, and she would like to absolve herself of guilt by demonstrating her support of the anti-apartheid movement. One afternoon, she and Vercueil go for a drive. They drive to the overlook of False Bay, and Vercueil buys some liquor and they drink together. Curren is already drunk, but Vercueil tries to convince her to drink more. Offended, Curren screams at him to leave, which he does. He also throws the keys into the brush, leaving her stranded there. He eventually returns, finds the keys, and drives her home.
One night, Mrs. Curren wakes up to find John in her kitchen. He asks about Bheki, and she tells him that Bheki was shot and killed. She urges John to abandon his cause and tells him that he will end up dead too if he continues. Curren doesn't believe that John was discharged from the hospital. She's convinced that he just left the hospital, stole some clothes, and found his way back to her house. He's still clearly injured, so she lets him stay in Florence's old room. While he's staying there, she catches him hiding things under the floorboards, so she calls Thabane, urging him to send someone to get John and bring him home.
The next morning the police come to her house asking about John. She says that everything is fine, but John is afraid. Promising not to let anyone hurt him, she tries to draw John out of the room peaceably. A female officer arrives at the scene and tries to coax Mrs. Curren away from the action. Realizing Mrs. Curren is sick, the officer tries to convince her to check into the hospital, but Curren refuses. Meanwhile, the other officers shoot and kill John. They wheel him out of the house on a covered gurney. Curren walks away from the house, ignoring the officer's pleas for her to check into a hospital, or at least return to her house. Mrs. Curren is disgusted by what has happened in her house and refuses to return as long as the police are there. She wanders the streets until she falls asleep on the sidewalk. She wakes up to little boys groping her and sticking sticks in her mouth. She's in excruciating pain. Somehow Vercueil finds her, but she still refuses to go home, so he carries her into a wooded area where they spend the night. They return to her house the next day and find that the police have ransacked it, looking for weapons or any evidence of further involvement with anti-apartheid groups. A detective comes and interrogates Curren, but she takes her pills and is barely conscious halfway through the interrogation. After he leaves, she calls Mr. Thabane to warn him.
From this point on, Mrs. Curren fades quickly as the cancer progresses. Her pain gets worse, and she has bizarre nightmares. Vercueil, who is caring for her now, still encourages her to commit suicide, but he also urges her to eat and cooks all of her meals. They start sharing a bed so that she can stay warm. One night, she wakes up extremely cold; she asks Vercueil if today is the day. Without a word, he climbs into bed and embraces her. Her final words to her daughter are that she can no longer receive warmth, and it seems that she will die in Vercueil's arms.
