In 1952, a semester before Ernesto "Fuser" Guevara is due to complete his medical degree, he and his older friend Alberto Granado, a biochemist, leave Buenos Aires in order to travel across South America. While there is a goal at the end of their journey - they intend to work in a leper colony in Peru - the main purpose is initially fun and adventure. They desire to see as much of Latin America as they can, more than 14,000 kilometers (8,700 mi) in just four and a half months, while Granado's purpose is also to bed as many women as will fall for his pickup lines. Their initial method of transport is Granado's dilapidated Norton 500 motorcycle christened La Poderosa ("The Mighty One").
Their planned route is ambitious, bringing them north across the Andes, along the coast of Chile, through the Atacama Desert and into the Peruvian Amazon in order to reach Venezuela just in time for Granado's 30th birthday on 2 April. However, due to La Poderosa's breakdown, they are forced to travel at a much slower pace, often walking, and don't make it to Caracas until July.
During their expedition, Guevara and Granado encounter the poverty of the indigenous peasants, and the movie assumes a greater seriousness once the men gain a better sense of the disparity between the "haves" (to which they belong) and the obviously exploited "have-nots" (who make up the majority of those they encounter) by traveling on foot. In Chile, for instance, they encounter a penniless and persecuted couple forced onto the road because of their communist beliefs. In a fire-lit scene, Guevara and Granado ashamedly admit to the couple that they are not out looking for work as well. The duo then accompanies the couple to the Chuquicamata copper mine, where Guevara becomes angry at the treatment of the workers.
However, it is a visit to the ancient Incan ruins of Machu Picchu in Peru that solidifies something in Guevara. His musings are then somberly refocused to how an indigenous civilization capable of building such beauty could be destroyed by the creators of the eventual polluted urban decay of nearby Lima. His reflections are interrupted by Granado, who shares with him a dream to peacefully revolutionize and transform modern South America, to which Guevara quickly retorts: "A revolution without guns? It will never work."
Later, in Peru, they volunteer for three weeks at the San Pablo leper colony. There, Guevara observes both literally and metaphorically the division of society, as the staff lives on the north side of a river, separated from the deprived lepers living across the river to the south. To demonstrate his solidarity, and his medical belief that leprosy is not contagious, Guevara refuses to wear rubber gloves during his visit as the head nun requires, choosing instead to shake bare hands and interact normally with the surprised leper patients.