John Riley, is an Irish-American officer in the US Army in 1846 just before the break-out of the Mexican-American War. His men have been imprisoned for attending mass with the local Mexicans and he sets out to free them and is promptly attacked by the US Army. They escape into the desert mountains and are captured by Mexican guerilla forces. Riley, wounded in the battle, is nursed back to health by a beautiful Mexican fighter, Marta. After forming a bond with the guerillas, Riley and his men elect to fight on their side. The 'San Patricios', as they are called, fight bravely, but after the US declares war on Mexico they are evetually taken prisoner. Riley is publicly beaten, branded and forced to watch as his men are hanged. Only when the US leaves Mexico is he finally freed from prison and allowed to journey deep into the Mexican countryside to be reunited with Marta.