The book Quincas Borba is about Rubião, a schoolteacher from Barbacena who inherits the fortune of a dying philosopher, moves to Rio de Janeiro, desires his friend's wife, goes mad convinced that he is Napoleon III, and returns to Barbacena, where he dies.
The novel opens with Rubião already rich, at a window of a house in Botafogo, and only then goes back to 1867. In Barbacena, he looks after the philosopher Quincas Borba for about six months; the philosopher expounds to him the doctrine of Humanitas and the maxim "To the victor, the potatoes," records his will, goes to Rio, and dies. The will makes Rubião universal heir, with a single condition: to care for the philosopher's dog, which has the same name as its master, "as if it were not a dog, but a human person."
On the train to the Court, Rubião tells the inheritance to a couple, Cristiano Palha and Sofia, and Palha starts guiding him: he recommends a lawyer and accompanies him on purchases for the house in Botafogo. Rubião falls in love with Sofia and makes her a bold proposal in a garden in Santa Teresa. He invests money in the Atalaia, the newspaper of the politician Camacho, becomes Palha's partner, and hears from Camacho the offer of a candidacy for deputy, until Palha dissolves the partnership on the pretext of a bank that, according to the narrator, did not exist.
After that, Rubião has his beard cut to match the bust of Napoleon III and starts to believe himself the emperor, and the mania spreads. Palha moves him to a small house on Rua do Príncipe and commits him to a sanatorium. Rubião flees with the dog to Barbacena, where he dies, and the dog dies soon after. The last chapter tells the reader to weep or laugh.