Set in the stark plains and sunbaked towns of Chihuahua in 1905, the tale opens with a pair of young men sent out on parallel tests of manhood, each equipped with a single cartridge in what locals call the prueba de cartrucho. Cruz González, Víctor Manuel Mendoza hopes to bring back a bear, while Miguel Del Campo, Pedro Infante faces a sterner demand from his father: if he doesn’t return with a deer, he must not come home at all. When Miguel shoots at a deer but stumbles and fires into the air, Cruz later finds him altered by the moment of failure. He learns that Miguel’s single cartridge has left him with no deer and no way to return. In a quiet, decisive act of camaraderie, Cruz offers his own cartridge to Miguel, promising to fetch another from home. The plan works for Miguel, who returns with a deer and earns his father’s pride, yet Cruz ends up with nothing to show and earns the scorn of his godfather, Don Rafael, Arturo Soto Rangel.
A decade passes, and Miguel travels to San Andrés, Chihuahua, where he seduces a young woman named Anna María, Susana Cora. News soon arrives that his father is ill, and he must return home, promising that he will come back. He leaves behind the letter that Cruz sent him as proof of the necessity to leave.
After a period of mourning, Miguel hosts a birthday celebration for his sister Cecilia, who is played with tender warmth by Cecilia’s portrayer, María Luisa Zea. Marco enters with flowers, but Miguel and Cruz deny him entrance, and Miguel warns Marco to stay away from Cecilia. Marco hints that one of them has committed sins in San Andrés, but he refuses to spell out the accusation, urging the pair to uncover the truth themselves. A letter arrives during the festivities announcing Anna María’s worsening condition, prompting Miguel to hurry to San Andrés. By the time he arrives, Anna María has died, and Miguel vows to care for her newborn daughter and to avenge the dishonor she bore.
The story intensifies as Marco provokes Cruz in a cantina, while El Yaqui’s intervention mangles the tension with a pistol discharge that defuses the moment. Cruz and Miguel then seek Don Rafael’s help to borrow a donkey to feed the baby, only to find Cleofas’ house in disarray—where Mariana reveals that they’ve been robbed and that Cleofas has been kidnapped. Marco later compels Cleofas to write a letter to Cruz, naming the man who is the father of his child, and ordering a meeting that night. Cleofas escapes and warns Don Rafael that Cruz is in grave danger, and the two men set out to find him. In a cruel ruse, Marco swaps himself into another man’s clothes to lure Cruz, but the ruse goes badly awry as the man is killed by those defending Cruz.
Marco’s machinations escalate when his gang tries to kidnap Cecilia, but he pivots to a different ploy that centers on a letter Anna María left behind for Cruz’s eyes only, revealing that she and Cruz shared a child. Cruz asks Cecilia to marry him, and she accepts, though Marco seizes the letter first. A confrontation ensues between Miguel and Cruz and one of Marco’s men, Chueco, who begins to expose the hidden letter; Miguel ends Chueco’s life to shield the truth, and Cruz accepts the blame to spare Miguel, with Don Rafael putting Cruz under house arrest as a consequence.
Miguel finally tracks down Marco, demanding the letter and shooting Marco before he escapes. The truth about Anna María’s baby—that Miguel is the father—emerges through Marco’s confession, fueling Cruz’s desire to kill his longtime friend. Yet mercy wins out: Cruz’s mother pleads with him to spare Miguel, and Don Rafael arranges a duel with each man allotted only one cartridge. As the duel unfolds, Cruz falters and fires into the air, and Miguel steps forward to lend him his own cartridge. The gesture recalls the original act of generosity Cruz showed as boys, a moment that would now determine their fates as men. In the end, Cruz cannot bring himself to kill Miguel and walks away with Cecilia, choosing honor, mercy, and a fragile future over vengeance.
