As the book shows, the central theme of The Slum is João Romão's ambition and what it costs those around him: the shopkeeper climbs to the door of a rich marriage and a title, while the tenement he raises grows, gets cleaned up, and pushes the poorer people into the rival tenement.
The narrator diagnoses the ambition in the first chapter and describes it as a disease: "It was no longer ambition, it was a nervous malady, a madness, a despair of accumulating." João Romão sells the egg he likes, steals stone from the quarry, calculates that the quarryman's seventy milreis "will come back to my drawer," and, after the fire, tears the bottles of money from Libório and, seeing the expired banknotes, regrets not having taken them sooner, with the phrase "It would have been an act of justice!" What he wants changes over the course of the book: the penny, the title Miranda receives, the neighbor's daughter and, finally, to be "a titled man of higher rank than Miranda."
Class difference shows in the design of the place: Miranda's townhouse looks down on the tenement, and the neighbor whom Miranda envies ends up as his son-in-law. The tenement, for its part, becomes aristocratic: it gains lamps, verandas, and a guarantee letter, and the residents who made the tenement grow go to the Cabeça-de-Gato.
The book also deals with how the environment acts on people, according to the narrator. Jerônimo arrives serious and thrifty, and the heat, the food, and the music of the land carry him away: "Jerônimo became Brazilian." Piedade, who used to tend the laundry, ends up drunk in the kitchen, and the narrator concludes about her daughter: "The chain went on." Slavery enters as both plot and era: it is what allows João Romão to deceive Bertoleza and then to be rid of her.