The book consists of twelve chapters, the first of which introduces the narrative proper and which is set exactly a year after the events. The following eleven chapters happen in a single day and follow the Consul chronologically, starting early on the morning of the Day of the dead with the return of his wife, Yvonne, who left him the year before, to his violent death at the end of the day. In contrast with the omniscient narrative mode of the 1940 version, the published novel "focus[es] each chapter through the mind of one central figure, no two sequential chapters employing the same character's consciousness".
The number of chapters was important numerologically, as Lowry explained in a letter to Johnathan Cape: there are twelve hours in a day (and most of the novel happens in a single day), twelve months in a year (one year elapses between chapter 1 and the end of chapter 12). Besides, the number 12 is of symbolic importance in the Kabbalah which, according to Lowry, represents "man's spiritual aspirations". Finally, "I have to have my 12", Lowry says, since he hears in it "a clock slowly striking midnight for Faust".
In the first chapter, set on 2 November 1939, Jacques Laruelle and Dr. Vigil drink anisette at the Hotel Casino de la Selva, on a hill above Quauhnahuac (an approximation of the Nahuatl name of Cuernavaca), and reminisce on the Consul's presence, exactly a year ago. His alcoholism is discussed and his unhappy marriage; that his wife came back to him is remarked upon as particularly striking. Their conversation over (they are to meet later again that night at a party), Laruelle walks down from the hotel into town through the ruins of a palace of Archduke Maximilian. Along the way he remembers spending a season with the Consul: Laruelle's family and the Consul's adopted family (the Taskersons, consisting of a poetic-minded patriarch and a set of hard-drinking sons) rented adjoining summer homes on the English Channel. Afterward Laruelle spent some time with the Taskersons in England but the friendship soon petered out.
Laruelle is scheduled to leave Quauhnahuac the next day, but has not yet packed and does not want to go home, spending his time instead at the Cervecería XX, a bar connected to the local cinema, run by Sr. Bustamente. At that bar, he is given a book he had borrowed a year and a half before from the Consul—an anthology of Elizabethan plays he had meant to use for a film on the Faustus myth. Playing a variation on Sortes virgilianae, his eyes fall on the closing words of the chorus in Marlowe's Doctor Faustus , "Cut is the branch that might have grown full straight...", then finds a desperate letter by the Consul to Yvonne, a final plea for her to return, interspersed with descriptions of alcoholic stupor and delirium tremens. Laruelle burns the letter. A bell outside sounds dolente, dolore to close the chapter.
Chapter 2 finds the Consul sitting at the bar of the Bella Vista hotel in Quauhnahuac at 7:00am on 2 November 1938, drinking whiskey the morning after the Red Cross ball, when Yvonne enters. The Consul has not been home yet and isn't wearing any socks (as is explained later, his alcoholism is so advanced he cannot put them on). Yvonne has returned to try and save their marriage, but the Consul appears stuck in the past and begins to talk about his visit to Oaxaca, where he went on a drinking binge after Yvonne left. In interior monologue Yvonne wonders if the Consul will be able to return from "this stupid darkness". The chapter had opened with an exclamation about a child's corpse being transported by train; the Consul explains that in such cases in Mexico the dead child always needs to be accompanied by an adult, leading to a reference to William Blackstone, "the man who went to live among the Indians", a reference the Consul will repeat later on in the day.
Yvonne and the Consul leave the hotel and walk through town, along the Palace of Cortés, Cuernavaca; they stop at a printer's shop window, their attention drawn by a photograph of a boulder split in two by the elements, an image Yvonne immediately recognises as emblematic of her marriage. On the way to their house in the Calle Nicaragua they stop at Jacques Laruelle's "bizarre" house, with the inscription No se puede vivir sin amar ("one cannot live without loving") on the wall, and Popocatepetl and Ixtaccihuatl come into view. The Consul tells Yvonne that Hugh is staying with him as well and is expected back from a trip this very day. As they enter the garden of their house a pariah dog follows them in.
Yvonne inspects the garden, which has fallen into chaos while she was away, and the Consul is making an attempt to keep up the appearance that he is dealing with his drinking problem. Throughout the chapter, hallucinations, memories, and imaginary conversations interrupt his train of thought, and he hears voices that alternately tell him all is lost and that there is still hope. Dr. Vigil had prescribed him a strychnine concoction which the Consul sips from continuously, all the while trying to resist the temptation to drink whiskey. While Yvonne is in the bathroom, however, he leaves the house to visit a cantina but falls facedown in the street, passed out, and is almost run over by an English driver in an MG Magna who offers him Burke's whiskey from a flask. While unconscious, memories of Hugh return to him, particularly his having forced Yvonne on him. Back at the house, he enters Yvonne's bedroom but their conversation is halted, in part by the temptation of the bottle of Johnny Walker he knows is on the patio and in part by hallucinations. An unsuccessful attempt at making love to her establishes his impotence and his despair; afterward, while Yvonne is crying in her room, he murmurs "I love you" to his bottle of whiskey and falls asleep.
Much of the chapter takes Hugh's point of view. Hugh arrives at his brother's home and it's understood that he's not wearing any of his own clothes. Because his clothes have been impounded, he wears his brother's jacket, shirt, and bag. He stores his news dispatch in his brother's jacket. References to the Battle of the Ebro are found throughout the chapter, as are mentions of Hugh's friend Juan Cerillo, a Mexican who was in Spain with Hugh. Hugh sees Yvvone at the Consul's home, it's obvious that Yvvone has some hold on his heart. In fact, an affair between the two is alluded to in the chapter. While the Consul is sleeping, Hugh and Yvonne rent horses and ride through the countryside, stopping at a brewery and then at the country estate of Archduke Maximilian, Emperor of Mexico, haunted by the memory of Maximilian and his consort Carlota, and of the Consul and Yvonne in happier times.
While Hugh and Yvonne are out, the Consul endures a "horripilating" hangover. The chapter begins with a vision of a man suffering unquenchable thirst; while the Consul inspects his garden (the Garden of Eden is referenced throughout, and a snake crosses his path) he finds a bottle of tequila he had hidden, and sees a newly placed sign: LE GUSTA ESTE JARDIN? QUE ES SUYO? EVITE QUE SUS HIJOS LO DESTRUYAN! He mistranslates this as "You like this garden? Why is it yours? We evict those who destroy!" As he is getting more drunk he has visions of the Farolito , a bar in Parian. He engages his American neighbour, Mr. Quincey, in conversation. Quincey obviously disdains the drunk Consul, who speaks of the garden of Eden and proposes that perhaps Adam's punishment was to continue to live in the Garden of Eden, alone, "cut off from God". Vigil (also hungover from the Red Cross ball) visits Quincey; he carries a newspaper with headlines of the Battle of the Ebro and the sickness of Pope Pius XI. He then visits the Consul, telling him to stay away from mescal and tequila. Hugh and Yvonne return, and the Consul wakes up from a black-out in the bathroom, slowly remembering the strained conversation during which it is decided that rather than accept Vigil's offer of a day trip to Guanajuato they will go to Tomalin, near Parian.
Hugh ruminates upon his career as sailor, journalist, and musician, while smoking a cigar. He imagines himself to be a traitor to his "journalist friends", somehow responsible for the Ebro, and comparable to Adolf Hitler as "another frustrated artist" and anti-semite. It is revealed that Hugh's signing aboard the S.S. Philoctetes was intended as a publicity stunt to promote his songs, which are to be printed by a Jewish publisher named Bolowski. Doubting his choice, Hugh attempts to escape his journey to sea but is thwarted by the Consul, who wires words of support for Hugh's choice to their aunt. Hugh remembers his time aboard the Philoctetes and, later, the Oedipus Tyrannus , revealing his naivete and bigotry. Back in England, Hugh finds that Bolowski has made "No effort [...] to distribute [Hugh's songs]". It is further revealed that Hugh cuckolds Bolowski, who raises charges of plagiarism against Hugh. Later, Bolowski drops the charges. Once again in the present, Hugh shaves the Consul, who is suffering from delirium tremens . The two men discuss literature and the occult; their discussion is intermingled with Hugh's ongoing inner monologue. At the end of the chapter, Hugh, Yvonne, the Consul, and Laruelle make their way to the home of the latter. On the way, the Consul receives a postcard from Yvonne, which she wrote the year before, days after she left him, and which has travelled around the world before reaching Quauhnahuac.
The four arrive at Jacques Laruelle's home, which features two towers that the Consul compares to both Gothic battlements and the camouflaged smokestacks of the Samaritan . Hugh, Yvonne, and the Consul go upstairs, where the latter simultaneously struggles to resist drinking and look for his copy of Eight Famous Elizabethan Plays . Yvonne wants to leave from the start, and she soon suggests going to the fiesta before they board the bus to Tomalin. The Consul stays behind as Hugh and Yvonne leave; once the two are gone, Laruelle rounds on him for coming only to drink. The Consul can no longer resist, and does so while Laruelle changes into his tennis clothes for a match with Vigil. They accompany each other down to the fiesta, where the Consul gets himself drunk at a cafe called the Paris while Laruelle tries to lecture him on his drinking. At the fiesta, more mention is made of the Pope's illness and the Battle of the Ebro. Eventually Laruelle leaves, although the Consul is not sure when; he ends up lecturing himself on his drinking problem. Now wandering around to avoid Hugh and Yvonne, he finds an unoccupied ride called the Infernal Machine and is pressured by a gang of children to take the ride. He loses all of his possessions on the ride, which the children gather and return to him. The Consul still has more time to waste, so he stumbles into the Terminal Cantina El Bosque, wherein he chats with the proprietor, Senora Gregorio, and has at least two more drinks. The pariah dog follows him inside but is scared off when he rises. Finally, he walks back outside to find Vigil, Quincey, and Bustamente walking together—they do not notice him—just as the bus to Tomalin pulls into the station.
The Consul, Hugh, and Yvonne travel to Tomalin by bus. A number of allusions and symbols are repeated: Las Manos de Orlac, the battle of the Ebro, cigarettes, the Good Samaritan, the number seven, et cetera. Along the way, Hugh notices a dead dog at the bottom of the barranca. During the trip, a pelado on the bus is noted, and Hugh and the Consul debate the meaning of the epithet. Hugh believes the term to mean "a shoeless illiterate" but the Consul corrects him, claiming that pelados are "indeed 'peeled ones,' the stripped, but also those who did not have to be rich to prey on the really poor". Further along, Hugh spots a man by the road, seemingly asleep. The bus stops and the man is found to be an Indian dying with his hat covering his face. No one helps the man due to a law that makes any such samaritan liable for "accessory after the fact". However, the pelado removes the Indian's hat, revealing a head wound and bloodied money. Nearby, Hugh and the Consul spot the recurring horse: "branded number seven". Leaving the peasant to his fate, the passengers reboard the bus. The Consul directs Hugh to look at the pelado, who is now clutching the "bloodstained pile of silver pesos and centavos": the pelado has pilfered the Indian's money, and has used it to pay his fare. The Consul, Hugh, and Yvonne take a pinch of habanero, and the bus rattles on toward Tomalin, where it arrives at the end of the chapter.
The Consul, Hugh, and Yvonne arrive at Arena Tomalin and take in a bullfight. The narrative references Munro Leaf's Ferdinand the Bull, as the bull does not care to participate in the occasion for much of the event. This chapter offers Yvonne's point of view, including her memories of the Indian that had been injured and the emotion that she feels when reflecting on the volcano, Popocatepetl. As the chapter continues, she reminisces about her childhood and early adulthood in multiple instances; for example, when she claimed to see her father come toward her in a hallucination, when she thinks about her mother’s death around the time of World War I, and when she discusses her life as an actress in Hollywood. Also, Yvonne dreams of the future she could have had, and would still like to have, with the Consul. Yvonne’s futuristic dreams included living with the Consul in solidarity and peace with nature. During the bullfight, Hugh decides to jump in and fight the bull while the Consul and Yvonne profess their love for each other in the crowd.
Told from the Consul's perspective, Chapter 10 opens with Geoffrey having drinks at the Salon Ofelia. He sits at the bar contemplating varieties of liquor while listening to Hugh and Yvonne banter as they swim nearby. They dress in separate changing rooms as the Consul continues to listen to their playful repartee, and they soon join him for dinner. Various landmarks, including the San Francisco Convent, the City Parish and the Tlaxcala Royal Chapel and Sanctuary, are mentioned as the Consul reads a tourist information booklet and remembers places he and Yvonne visited in happier times of their past. The Consul leaves early after arguing politics with Hugh and lashing out against Hugh and Yvonne's concerns about his drinking.
Hugh and Yvonne leave Salon Ofelia in search of the Consul. They walk in the shadow of the two volcanoes, Popocatepetl and Ixtaccihuatl and come to a point where they must choose one of two paths. Told from Yvonne's perspective, they choose the main path, which they believe the Consul would have taken, since it passes by two cantinas on the way to Parian. Their progress is hindered by a growing thunderstorm and there are numerous references to constellations, such as Orion and the Pleiades. Yvonne is trampled by the horse with the number 7 branded on its leg and imagines seeing her dream house in Canada burn down as she dies.
The final month of a year, the final number on a clock, and the final chapter of Under the Volcano, told again from the Consul's point of view. He is in the main barroom of the Farolito, which is located at the foot of and seemingly under the volcano Popocatepetl. He does not realise that Hugh and Yvonne are looking for him. Diosdado, also called The Elephant, hands the Consul a stack of letters he has had, which were written by Yvonne and sent to the Consul throughout the past year. The Consul gets into a disagreement with the local police chiefs. They push him outside of the bar and out of the light, where they shoot the Consul and throw him off the edge of the ravine that the Farolito is built atop of. The shot startles a horse which runs off and tramples Yvonne.