Reflecting the author's political development, the book expresses Socialist ideas and promotes workers' organizing for class struggle - specifically, in the harsh and exploitive world of the cocoa plantations.

Cacau ends with the hero renouncing the opportunity to marry the landowner’s daughter and, instead, setting off to join the class struggle in Rio de Janeiro.

He suffers long hunger and is apprehensive of the city until encountering a good-hearted guard named Roberto in front of a big bakery, who gives him some bread.

With the money that these two new friends give him, the young man manages to catch the train to Pirangi and, after a long journey and numerous events described in detail, he arrives at his destination.

There, he embarks on the course of hard work and befriends the carpenter Colodino, as well as fellow workers João Grilo, Antonio Barriguinha and Honório, who are mostly Blacks or Mulattos.