In late 19th century Mexico, Federales capture Quintero (Fernando Rey), a revolutionary who attempts to rally those opposing the dictatorship of President Díaz. Before going to prison, Quintero gives his lieutenant, Maximiliano O’Leary (Reni Santoni), $600 (equivalent to $17,000 in 2019) with which to continue the cause. Bandit chief Carlos Lobero (Frank Silvera) demands that the money be used for guns and ammunition, but Max instead crosses the border in search of Chris Adams (George Kennedy): a legendary, American gunman whom his cousin had told him about. Max finally finds the laconic Chris, witnessing him free a man from a rigged trial, first by using his wits, then with the famed hair-trigger skill as a gunfighter.
Chris agrees to mount a rescue of Quintero and uses $500 of Max’s money to recruit five highly trained combatants: Keno (Monte Markham), a horse thief and hand-to-hand combat expert (whom Chris saved from hanging); Cassie (Bernie Casey), a brawny but intelligent former slave, who can handle dynamite; Slater (Joe Don Baker), a one-armed, sideshow sharp-shootist; a tubercular wrangler called “P.J.” (Scott Thomas), and Levi Morgan (James Whitmore), an aging family man who is doubtful of his worth, despite his incredible knife-throwing skills.
En route to Mexico, the motley band of Americans becomes less mercenary when observing the brutal treatment of the peasants. Their journey is marked by encounters with a political prisoner’s little boy, Emiliano Zapata (Tony Davis) and a pretty peasant girl, Tina (Wende Wagner), who falls in love with P.J. When Lobero learns that Max did not buy guns with the $600, he refuses to allow his men to take part in Quintero’s rescue. Realizing that he needs support, Chris frees a prison gang that includes Zapata’s father, then trains them in military tactics.
Despite their superior fighting skills and strategy, Chris’ men are outnumbered and their valiant effort to free Quintero appears doomed. At the last moment, 50 of Lobero’s bandits, having slain their leader for his lack of patriotism, thunder onto the prison grounds and turn the tide of battle. Of the original seven, only Chris, Max and Levi survive. Before riding home, Chris and Levi leave behind the $600 the peasants had collected.