The bad Barton boys: Blake, Leroy and Gabby, rob a train and shoot a guard. Luke Smith, known as "Whispering" to some for his quiet, sly ways, is a detective for the railroad, sent to investigate. Murray Sinclair, an old friend of Smith's, is in charge of the railroad's wrecking crew. He's glad to see Smith, who shoots Leroy and Gabby and is saved when a bullet is deflected by a harmonica in his pocket, given him long ago by his sweetheart Marian, who is now Sinclair's wife. It saddens Smith to find out that Sinclair might be in cahoots with Barney Rebstock, a rancher with a bad reputation. Rebstock has been hiding the remaining Barton brother, Blake, who is tracked down by Smith. Whitey DuSang is a hired gun for Rebstock, who wants to see Smith dead. When the railroad's boss gives Sinclair an order, Sinclair rebels and is fired. Rebstock hires him to pull off a string of daring train holdups. Can legendary railroad detective 'Whispering' Smith foil the robbers, save his old friend Sinclair and save the day?