An ageless freedom fighter against colonization, Matigari buries his many weapons under a fig tree. After killing his oppressor, Howard Williams, he "girds" himself with a "belt of peace," vowing to resolve conflicts nonviolently. Before returning home, however, he wants to find his people.
Having lived in the wilderness for four decades, Matigari marvels at the changes he sees in his country; people drive their own cars, and the city has grown considerably. Knowing his countrymen and women are workers, he decides to start looking for his people at a factory. However, he is appalled when he discovers that orphaned children pay an entrance fee to rummage through the garbage at a dump, their only way to find necessary food and supplies. One boy fights another for a shoelace, so Matigari intervenes and chases the bully away. The other boy, Mũriũki, then leads Matigari to a scrap yard, explaining that the children use old cars for shelter. When Matigari tries to speak with the children, they throw stones at him, as most adults beat the children.
A factory worker named Ngarũro wa Kĩrĩro comes to Matigari's aid, bringing him to a restaurant where he can recover from his injuries and shelter from the heat. On the way, Matigari tells Ngaruro the story of how he killed Settler Williams for oppressing and exploiting the people. After years of building things for Settler Williams yet living in poverty, Matigari tried to kill Settler Williams, but the colonist's servant, John Boy, attacked Matigari. Matigari escaped into the wilderness with Settler Williams hunting him. Eventually, however, Matigari was able to kill Williams in the mountains. Hearing the name Williams, Ngarũro mentions that the exploitative factory owners are Settler William's and John Boy's sons.
Matigari, Ngarũro, and Mũriũki reach a bar, and Ngarũro returns to the factory to lead a workers' strike for better wages and conditions. Matigari orders food and drink but does not eat anything. Instead, he is propositioned by a prostitute called Gũthera, who hides in the bar from two policemen harassing her. However, the policemen catch her and set their dog on her. Matigari intervenes without fear or weapons, admonishing the policemen for threatening innocent people. His courage frightens the two officers, who let Gũthera go.
Back at the bar, Gũthera explains that she hates the police for killing her father, who was arrested because he was a patriot fighting for independence. As a way of thanking Matigari for saving her life, she decides to stay with him and help him find his people. The group eventually reaches an enormous plantation and mansion that Matigari claims is his. The sons of Settler Williams and John Boy refute and mock Matigari, demanding to see a title deed proving Matigari's ownership of the house. Matigari refuses to cooperate and is arrested when he tries to open the gate.
Matigari finds himself in a dark cell with other inmates who were arrested for crimes they committed out of desperation or for "spreading communism." Matigari shares his food with them, which reminds the drunk inmate of the Last Supper, a scene from the Bible. After exchanging their stories, the inmates are released under mysterious circumstances, which leads to Matigari becoming a legend who is compared to an angel and even God himself for being able to escape prison and stand up to the police.
People across the country share Matigari's story, adding more fantastic details and glorifying him as a saint or suggesting he is the second coming of Jesus. However, as Matigari roams the country asking where he can find "truth and justice," the people reject and criticize him. He visits ordinary people until an old woman tells him to go to the students, as they are also searching for the truth. However, the student and the teacher who shared Matigari's cell are too afraid to help Matigari. He then decides to go to the church to ask the priest for advice, but the priest, a puppet of the government, also refuses to help. Instead, he suggests Matigari attend a meeting later that day and talk to the Minister for Truth and Justice himself.
At the meeting, observed by representatives from a number of Western countries, the Minister for Truth and Justice instates several policies that are human rights violations, like ending workers' strikes, banning gatherings of more than five people, and declaring that people discussing communism will be arrested. Then, the prisoners who escaped with Matigari earlier are presented to the public and convicted through "instant justice" by judges and a jury of loyal to the government. The only man who is not convicted turns out to be an informer.
Matigari confronts the Minister for Truth and Justice directly, calling his government unlawful and accusing him of exploiting the majority. The Minister villifies Matigari, calling him a madman and sending him, Ngarũro, and the other prisoners to a mental asylum. The people at the assembly, however, start singing songs about revolution, causing a commotion.
After the meeting, the government imposes strict laws to eradicate the revolutionary spirit Matigari inspired. At the mental hospital, Matigari decides to dig up his weapons and fight the oppressors with violence, as he has realized that words and reasoning alone cannot bring about change. With the help of Gũthera and Mũriũki, he escapes. On the way to the tree where Matigari's guns are buried, the group manages to steal a Mercedes-Benz from the Minister's wife and her lover. The news on the radio informs them of Ngaruro's death.
However, the informer discovers Matigari and the police chase him to John Boy's house, surrounded by a crowd of workers, government officials, and "VIPs." Matigari drives through the front door and lights the house on fire, inspiring other workers to destroy nearby cars and houses. Gũthera, Mũriũki, and Matigari then run away from the police. Just before reaching a river, which Matigari says will lead to their safety, he and Gũthera are shot. Their bodies fall into the river and are never found, leading to new myths and legends about Matigari. Mũriũki, the only clear survivor, eventually reaches the fig tree, digs up Matigari's guns, arms himself, and starts to sing the song of victory.
