Don Segundo Sombra is the story of an orphaned boy named Fabio who has spent most of his early years in a small town living a somewhat rootless experience. One day he meets a gaucho named Don Segundo Sombra, the character who gives the novel its name, and is so impressed by the gaucho's quiet dignity that he decides to take him up as a mentor. He seeks out to the ranch of Sombra's current employer in order to obtain work alongside the gaucho. In the years that follow, he learns much from Sombra and travels widely around Argentina. The narrative details some of the adventures he encounters along the trail, including time spent among other gauchos.
Though there is some overlap, there is also a marked difference between the gaucho life as portrayed in Martin Fierro and that seen in this novel. Martin Fierro may have his honorable aspect, but he is basically an outlaw and a killer. While Sombra has its share of drawn knives, bloodshed is generally avoided. The one death that does result is portrayed as a tragedy and waste, without the outlaw romanticism of the older book.
What is most striking is the extent to which the gaucho is a civilizing influence. It is through Don Segundo that Fabio learns about courage, honesty and loyalty--values it is implied that he would not have picked up had he stayed with the distant relatives with whom he is staying at the beginning of the novel.
Like Martin Fierro, the novel's language draws heavily from Argentine, especially gaucho, manners of speaking, though written in a more natural and readable style. As with much gauchesque literature, the gaucho Sombra serves as symbolic of national character. Unlike those older works, Sombra was written when the real-life gauchos had begun to disappear and so reflects the shift of the gaucho from reality to myth, a lost emblem of the forging of personal and national adulthood.