The Three Mesquiteers are forced to track down a train robbery ring after some of the gang hijack their truck for a getaway and the police conclude they are part of the gang, an identification which is just fine with the gang's nameless chief.
Two men are rivals for the same girl. When she finally agrees to marry one, the other--appearing to be magnanimous in defeat--presents his former rival with a beautiful German Shepherd dog as a wedding present. It turns out, however, that he had an ulterior motive--he had trained the pooch to allow absolutely no one to get near the young woman. Complications ensue.
