An American adventurer competes with the famed Mexican bandit to recover a lost gold shipment.

In 1915, in the arid mountains of western Mexico, American mercenary Tom Bryan and revolutionary Col. Juan Castro hide behind a wall of bagged gold, as a group of armed soldiers close in. While awaiting the inevitable confrontation, Tom recalls how he and Juan ended up together: In a small town, Tom, who owns a powerful machine-gun called "La Cucaracha," helps Juan rob a bank, the spoils of which are to go to Pancho Villa's revolutionary army. Later, Tom delivers the money to one of Villa's aides, who tells him about a planned holdup of a gold shipment in Santo Tomas. At first, Tom refuses to become involved, as he has grown tired of the rugged rebel life, but changes his mind when he learns that Juan is running the operation. In Santo Tomas, meanwhile, Juan inspects the site of the robbery, then rides to the rebel hideout. There he confers with Capt. Pablo Morales and Pablo's sympathetic wife Laria about the train heist. Because of a past wartime experience with Pablo, Juan is mistrustful, but Laria vouches for her husband's loyalty. Laria then introduces Juan to Ruth Harris, an American whose mine owner father was recently killed by government soldiers. Laria has befriended Ruth, an idealistic schoolteacher, and asks Juan to take her north with him after the robbery. After Juan promises to consider Laria's request, he, Pablo and their rebel band lie in wait for the gold-bearing train. As hoped, Tom is on board the train, carrying La Cucaracha in a bass fiddle case. Once the holdup begins, Tom grabs his machine-gun and joins the rebels in killing the soldiers guarding the gold. The gold is taken, and later, the rebels load the bags onto donkeys, along with some dynamite, and prepare to deliver them to Villa. While camped, Tom meets Ruth and is immediately attracted. After Juan gives Ruth permission to accompany him, she thanks Tom, assuming that he had put in a good word for her. Tom, in fact, does not think that Ruth should make the hazardous journey and disparages her commitment to the revolution. In turn, Ruth questions Tom's claims that he is only in it for the money and, when he insists that he could care less about Villa, denounces him. Soon after the rebels depart, soldiers, guided by Indians, discover the abandoned camp and pick up the rebels' trail. Juan, meanwhile, leads the rebels across the rough, hilly terrain, reminding Pablo repeatedly that the gold belongs to Villa. One evening, Tom gives Ruth some perfume, and touched, she tells him that she prefers his "real," non-mercenary self. When Tom scoffs and declares that he is hoping to make a fortune off the revolution, however, Ruth again condemns him, but allows him to kiss her. Later, from a cliff, Juan spots the soldiers approaching and orders Pablo and some of the other rebels to take the gold to Villa, while he and a few others stay and fight. Ruth insists on fighting with Juan, but to her disgust, Tom decides to go with Pablo and keep an eye on the gold. Soon after, however, Tom hears the sounds of a gun battle and races back with La Cucaracha to help Juan. Tom and Juan defeat the soldiers, and Ruth apologizes to Tom. Later, at the place where the gold is to be delivered to Villa, Juan waits for both Villa and Pablo. Instead, Pablo and his men take Juan and the others prisoner, explaining that years earlier, while he and Juan were soldiers in another revolutionary army, Juan stole some gold for the then unknown Villa and forced Pablo to go into hiding. As revenge, Pablo intends to steal all the gold, sure that Villa will not come. Juan provokes Pablo into a fight, and during the commotion, Tom grabs La Cucaracha and subdues Pablo. Tom then unarms all of Pablo's men and forces Pablo to take Ruth and head for the safety of Tampico. Then, with no sign of Villa, he orders Juan and the remaining men to push on toward Yaqui country. Although Juan curses Tom for his betrayal, Tom offers to split the gold with him but is refused. Quickly, the men going toward Yaqui country begin to slip away from the mule train, driven by fear and thirst. Pablo and Ruth, meanwhile, run into another band of soldiers, and Pablo strikes a deal with the commandant, offering to lead the soldiers to Juan and Tom, in exchange for his freedom. After condemning Juan and the commandant, Ruth is sent north, while Juan guides the soldiers into Yaqui country. By the time the soldiers catch up to them, Tom and Juan are alone with the gold and the dynamite, high in the hills. Tom gives Juan a gun, and together they stack the gold bags to create a barrier. When Pablo is sent to negotiate with Juan and Tom, Juan shoots him, instigating a gun battle. Juan and Tom down many soldiers before their ammunition runs out, then plant the dynamite and retreat farther into the hills. While awaiting their fate, Juan assures Tom that Ruth was in love with him and insists that he was never interested in the gold. A soldier then sneaks up and kills Juan, but is in turn slain by Tom. Now alone, Tom lights the dynamite, and in the ensuing explosion, the gold as well as the soldiers are buried in a rock slide. Later, Tom, determined to keep the buzzards away, buries his friend's body under a pile of rocks.