From what the book shows, The Slum puts under the lens the unscrupulous ambition of João Romão, the way he gets rich from the work of others, and the distance between the townhouse and the tenement. The narrator does not preach: he sets out the facts in sequence and lets the reader see who rises and who pays.
Ambition is described as a disease: "It was no longer ambition, it was a nervous malady, a madness, a desperation to accumulate". To accumulate, João Romão forges Bertoleza's letter of freedom, raises the little houses with stone stolen from the quarry and, after the fire, takes Libório's bottles of cash.
The labor of others supports every step. The tenement rises on Bertoleza's savings and the laundresses' rent. When he hires Jerônimo for seventy milreis, João Romão calculates: "My seventy milreis will come back to my drawer. It all stays in my house!". From Miranda's window, he looks down with "looks of contempt upon that sensual rabble, which had made him rich". And, the morning after the fire, it is Miranda who says the homeless "have nothing to lose" while the owner is busy with the insurance.
The distance between the townhouse and the tenement is in the layout of the place, with the wall of broken glass between the two. Miranda, at the window, says: "Ah, scoundrels! What I should do is throw them out of here, like mad dogs!". In the end, the neighbor Miranda envied becomes his son-in-law, and the tenement "became aristocratic", with a letter of surety required at the entrance and higher prices, and many of the old lodgers desert to Cabeça-de-Gato. Slavery enters as plot and period, in the false letter, in the return of Bertoleza and in the abolitionist commission at the end.