The Phantom Plainsmen(1942)

In 1937 the life in out West has not changed much. The boys are working at the Wyoming ranch of Captain Marvin herding horses which he sells to Kurt Redman. Marvin will not sell any horses to any army, but the boys find out that Redman is a German agent shipping the horses directly to the Third Reich. When Marvin tries to stop Redman, his son Tad, who is studying medicine in Germany, is arrested and held hostage. Marvin must fire the boys as the sneaky German agents take over the ranch, but the boys will not give up their attempt to stop them.