It is 1912, and groups of Mexican revolutionaries have been attacking towns on both sides of the Mexican–American border. The most powerful of these groups is led by a former Mexican army general, Héctor Cordoba [Raf Vallone]. When a surprise attack results in six cannons falling into Cordoba's hands, the United States government puts General Pershing [John Russell] in charge of seeing that the cannons will never be used against the American people. Pershing turns to Captain Rod Douglas [George Peppard], instructing him to gather a group of men to take part in the dangerous mission into Cordoba's territory.
The first man to sign up is Jackson Harkness [Don Gordon], a soldier who has worked with Douglas before. At the beginning of the film, Harkness has to stand by and watch as his brother is tortured and killed by Cordoba. Douglas ordered him not to step in because they were undercover as sympathizers in the enemy camp and could not afford to make their true intentions known. As a result, Harkness vows vengeance on the captain and will not leave his side until the opportunity presents itself.
The next two men that Douglas chooses for the operation are Andy Rice [Pete Duel] and Peter [Nico Minardos], who have just broken out of the army jail when Douglas arrives with the orders for their release. The captain now has all of the men that he feels are necessary for getting the job done. However, a Mexican lieutenant, Antonio Gutierrez [Gabriele Tinti], who holds a personal grudge against Cordoba, approaches him and demands to be part of the operation. He tells Douglas that he knows a woman, Leonora Cristobal [Giovanna Ralli], who, for her own reasons, wishes to see Cordoba dead. If the captain includes him in the mission, she will help them by working her way into Cordoba's confidence and getting him alone so that he will be vulnerable when they make their move.
Antonio and Leonora arrive at Cordoba's camp first. Leonora learns that the Mexican government wants Cordoba captured alive, betrays Antonio and informs the bandit leader of his intentions, hoping that he will reward her for what she has done by allowing her to get closer to him, giving her the opportunity to kill him herself.
When Douglas, Andy, Peter, and Harkness arrive at the camp, posing as sympathizers, they hear of what Leonora has done and decide that they have to act quickly. Douglas starts a fight with one of the Mexican men, so as to be put in jail, where he can help Antonio to escape. That night, Andy, disguised as a Mexican guard, breaks both of the men out of jail so that the operation can proceed. Douglas goes to Cordoba's room, where he finds him alone with Leonora. She betrayed Antonio but she still did the job she was supposed to do. Meanwhile, Jackson and Peter turn the cannons on the camp and begin to fire, while Andy and Antonio shoot flares into the buildings. Chaos ensues and the group of men, along with Leonora and their prisoner, Cordoba, attempt to ride out of the camp. Peter, Antonio, and Andy are killed in the process, and Cordoba is wounded.
The next morning, miles away from the camp, the diminished group stops to rest. When Douglas goes off by himself, Harkness sees his opportunity to avenge his brother. He follows the captain, demands that he turn around, and draws his gun. As Douglas walks unflinchingly toward him, however, he is unable to shoot and, instead, punches him. All now forgiven, the two men walk back to where Leonora waits. Cordoba has died from the wound he received the previous night. They are not able to bring him back alive, as the government had wanted, but the cannons were destroyed and their mission is complete.
