During the Mexican Revolution, a peasant named El Vasco starts a revolt in his town of San Bernardino by killing the army colonel in charge. Rebel leader and self-appointed General Mongo soon arrives on the scene and hires El Vasco for his revolutionary gang. However, Mongo is more interested in gaining fortune for himself than for his country. Yodlaf Peterson, a Swedish mercenary arrives in Mexico to sell guns to General Mongo. The safe containing the money is locked and only Professor Xantos knows the combination. Xantos is the leader of a student counter-revolution that opposes violence, and is held in a prison by the United States army just across the border in Yuma, after he tried to find funding from the US and did not agree to give the monopoly of his country's entire oil wealth in return. El Vasco dislikes the suit-wearing Peterson and calls him a "penguin", but at the suggestion of General Mongo, the two reluctantly join forces and set out to capture Xantos. Their task is made harder by the American army and a wooden-armed American named John, who wants to exact revenge on Peterson, his former business partner. Peterson had left John to die after he was crucified, and his pet hawk had to peck his right hand off to save him. In order to maximize their personal gain and to support Xantos' rebel fighters, El Vasco and Peterson have to doublecross Mongo. They receive help from Lola, the leader of Xantos' rebel group, who El Vasco falls for, and her group of young revolutionaries. El Vasco and Peterson free Xantos by infiltrating the US Army camp, setting off a series of explosions, and escape with Xantos in an old army truck, but the one of the truck's wheels becomes unhinged and the three men end up swimming across the Rio Grande river to safety back in Mexico. El Vasco, Peterson and Xantos are safely reunited with Xantos' followers, known as the Xantistas, hiding out in a cave. But Mongo captures several of the Xantistas and threatens to execute them if Xantos does not surrender. El Vasco and Peterson team up with Lola and the surviving Xantistas to launch a climatic attack on San Bernardino and they defeat Mongo's army in a climatic gun battle and Mongo himself is killed by El Vasco. Afterwords, when El Vasco brings Xantos to the bank vault to have him open the safe, the safe reveals to not hold any gold, silver or cash, only the "true wealth" of the community which include a few ears of corn. As Peterson prepares to leave town, John reappears and mortally wounds Xantos and prepares to kill El Vasco, only for Peterson to turn the tables on his enemy by blowing up a fuel tanker train along with John. When news arrives that a large Federale counter-revolutionary army is marching onto San Bernardino, Peterson decides to leave aware that the Xantistas are outnumbered and outgunned, but El Vasco urges him to stay and fight. Peterson declines and rides away, but at the very last minute, unwilling to allow the Xantistas be all killed by the Mexican army, Peterson rides back into town to join the revolutionaries in the imminent battle.
Compañeros was released in Italy in December 1970. The film grossed a total of 1,451,782,000 Italian lire domestically in on its release in Italy. It was released in West Germany in 1971.
