In a fort on the Frontier, the 7th Cavalry keeps a vigilant watch over the Wild West. It is a time of peace with the local Natives, and all is well, thanks - at least in the colonel's mind - to the regiment's lucky mascot. But when that dog mysteriously vanishes and bad luck begins to plague the unit, it becomes necessary to find a replacement. The colonel picks Rin Tin Can for the job ... and then the real trouble begins!