Carl Schaffner (Rod Steiger) is a widowed British businessman, born in Germany, who flees to Mexico with the police hot on his heels after stealing company funds. He has a fortune stashed in a Mexican bank to keep it out of reach of the British authorities. While traveling by train, Schaffner drugs and switches identities with fellow train passenger Paul Scarff (Bill Nagy), who looks like him and has a Mexican passport. He throws Scarff off the train, injuring Scarff. As part of the ruse, Carl is forced to take possession of Scarff's dog. The plan seems foolproof, but it backfires when Carl, discovers that Scarff is a wanted political assassin. Carl tracks down Scarff, who is incapacitated by his injuries, and gets back his original passport. Carl arrives in Mexico and is captured by the local police, who mistake him for Scarff. Carl has to reveal his true identity to the local police, but at first he is not believed. He tells the Mexican police where to find the real Scarff and they pass the information to the American police, and Scarf is killed when they go to arrest him. The local Mexican police chief, who at first seemed amenable to Schaffner’s approaches to bribe him, connects with Scotland Yard inspector Hadden. They conspire to keep Schaffner trapped in the Mexican border town of Katrina, and then will try to get him to cross the bridge into the U.S., where he can be apprehended. The misanthropic Schaffner has by now grown attached to Scarff's pet spaniel. He is tricked by Hadden and the police chief into having to cross the dividing line of the bridge to recover the dog. Schaffner realizes what is happening, but is determined to get the dog back. He is accidentally killed when he tries to run back across the border and a police car knocks him down. The realization of his own humanity has cost the cynical, friendless Schaffner his life.