Devil on the Cross is set in the postcolonial landscape of 1980s Kenya. Although, at this point, the country had its own legislature and government, the influence of international culture and currency still played a large role in the daily lives of everyday citizens, something that author Ngũgĩ wa Thiong'o reviled and saw as a residue of colonial oppression.
The text opens with a Gĩcaanndĩ player in Ilmorog, a fictional rural outpost in Kenya. He says that he is initially reluctant to tell the story that follows, but a divine intervention and the collective will of the people urges him to do so anyway. Jacinta Warĩĩnga is the main character of this story, and is introduced beginning in the second chapter. Warĩĩnga has recently lost her boyfriend, been fired for rejecting the advances of her boss, and feels completely despondent. She decides to return to her parents in Ilmorog, but suddenly attempts suicide while at a bus stop. Even so, an unknown man saves her before she is able to complete the deed. She tells him her life story—one of being impregnated at a young age as a sugar girl and being forced to abandon all of her childhood dreams and ambitions—and he is moved by her experiences. In responses, he offers her an invitation to a party called "The Devil's Feast," a competition in "modern theft and robbery." He tells her that the conditions that caused Warĩĩnga to be exploited will be explained if she goes to the Feast, but Warĩĩnga is uncertain if she wants to go, since the Feast is advertised using demonic language. Coincidentally, however, the Feast is being hosted in Ilmorog, where she is heading to anyway to see her parents.
To get to Ilmorog, she boards a rundown matatũ, driven by a greedy and unctuous man named Mwaũra. While on the matatũ, Warĩĩnga meets a wide array of characters, each of whom tells their life story and describes what is bringing them to Ilmorog. One such person is Mũturi, a handyman who is traveling to Ilmorog to look for work after being fired from his last job (he went on strike and asked for a livable wage from, as it turns out, the same person who fired Warĩĩnga). Also on the bus is Wangarĩ, a woman who fought for Kenyan independence during the Mau Mau Uprising but who was arrested for vagrancy while looking for work in Nairobi (she is on her way to Ilmorog to cooperate with the police and point out thieves and robbers, so as to avoid her own charges). Also present is Gatuĩria, a well-educated and polite student from the university who studies local music, on his way to Ilmorog to see the Devil's Feast and try to convince himself of the truth of old folktales (which tell of demons and the like that, up until this point, Gatuĩria has never seen). Finally, there is Mwĩreri wa Mũkiraaĩ, a reticent and wealthy businessman who speaks only to talk about his faith in theft. Theft, Mwĩreri wa Mũkiraaĩ claims, is what makes a country developed; the only thing that keeps a country beneath another on the global stage is its relative ineptitude to grab and steal in places where its people have not worked.
The matatũ ride to Ilmorog sees wide-ranging and deep discussions of the nature of good and evil, as well as a discussion of the post-colonial conditions that have empowered local tycoons and compradores. Even though Kenyans fought to earn their independence, and even though the people who are in power at the present moment are nominally Kenyan (e.g., by race, clan), they often serve at the feet of foreign masters and allow themselves to be manipulated by foreign money. In some ways, such conditions are even worse than colonial conditions because they keep the imperialistic truth hidden from the people. The consensus on the matatũ is that the Devil's Feast will host many of the thieves and traitors who enable the post-colonial system of capitalism, so each passenger is curious to go for their own reasons. Mwĩreri wa Mũkiraaĩ, however, informs them that the Feast is not in fact hosted by the Devil, but rather by a local association of thieves and robbers; the card that Warĩĩnga received, he says, is a forgery made by ideologue college students.
Upon arriving at the Feast, Warĩĩnga and the others bear witness to a series of barbaric, inhumane, and exploitative proposals by a series of businessmen and tycoons. Each tycoon talks about how they have exploited the people and earned vast fortunes, not just to share information and techniques among thieves but also to impress a delegation of foreigners that is to crown a winner. Their suggestions range from selling bottled air to peasants to selling human organs to the wealthy in order to prolong their lives. Mwĩreri wa Mũkiraaĩ takes the stage and recommends that the organization of thieves and robbers drive the foreigners out of Ilmorog so as to take a greater cut of the riches for themselves, but a commotion breaks out, and the emcee and others reject his idea.
Warĩĩnga and Gatuĩria choose to stay as spectators, while Mũturi and Wangarĩ, frightened by what they have heard, choose to lead a revolt of the peasants and call the police to capture those present at the Feast, respectively. Meanwhile, Warĩĩnga and Gatuĩria get lunch, and Warĩĩnga tells Gatuĩria of how her life was ruined when she became the sugar girl of a Rich Old Man from Ngorika, which led her to try and take her life several times, only to be rescued each time. Upon returning to the cave, Mwaũra tells Warĩĩnga and Gatuĩria of the other two's plans, and he also tells Warĩĩnga that it was Mũturi who rescued her each time from suicide. Another man takes the stage and begins to talk, but Warĩĩnga flees and falls asleep against a tree. While she is asleep, the Devil comes to her and exposes the truth of the entire colonial and imperial operation, rooted in the shared evil of capitalist exploitation, and invites her to take part in it. She rejects him, and wakes to find Gatuĩria watching over her. He tells her that Wangarĩ brought the police, but was herself captured and detained on the behalf of the wealthy, whom the police really work for. Soon after, Mũturi arrives with a horde of neighborhood laborers, university students, and workers, who walk on the cavern where the Feast is occurring. They stop the festivities and cause all the thieves to run away, but then are violently suppressed by the police and their associated auxiliary forces.
Two years pass. Warĩĩnga and Gatuĩria are to be married, and through extensive and costly preparation, she has satisfied her childhood fantasies of being a mechanical engineer. She has also become proficient in self-defense, followed the ideals of Marxism, and has held on to a pistol that Mũturi gave her at the cave in secret. Meanwhile, Gatuĩria has completed his musical composition on the history of Kenya and is to present it, along with his new girlfriend, to his father in Nakuru. Warĩĩnga leaves Nairobi with her man to go and meet his parents, but as she does so, she is also despondent because her old boss—with the support of businesspeople from America, Germany, and Japan—has purchased the garage where Warĩĩnga works so that he can wreck it and develop a traveler inn on the site.
Gatuĩria and Warĩĩnga first meet with her parents, and her mother gives the couple her blessing. Afterwards, they go to Nakuru, where Warĩĩnga sees that her boyfriend's family is one of the very tycoon families she has grown to loathe since the Feast—and, in fact, many of the people who spoke at the Feast are present at Gatuĩria's house. Moreover, she discovers that Gatuĩria's father is in fact the Rich Old Man from Ngorika who impregnated her and then abandoned her as a girl. Warĩĩnga restrains herself at first, but as the father continues to advance on her and even threatens her, she snaps. She shoots Gatuĩria's dad and a few other visitors from the Feast, claiming that she will stop them from ruining others' lives and continuing the systems of oppression they perpetrate. Gatuĩria is left standing, uncertain whose side to take, and as the novel ends, Warĩĩnga walks away from the house without looking back.
