The bounty hunter known as Montero learns that his brother is to be hanged for murder in the New Mexico town of Alamagordo. A single clue left by the dead man -- the two letters 'MA' -- ensures that a guilty verdict is inevitable. The two brothers had parted some years before under hostile circumstances, but Montero is convinced that Mace Gifford would never shoot a man in the back. He plans an ingenious escape but saving his brother's neck is only the beginning; he has to find the real killer.