Trying to acquire all the land in the Rio Perdido valley, Chick Mahoney (Don Harvey), Jed Mahoney (Myron Healey) and their gang, working under the orders of the mother of the Mahoney boys, Mrs. J.G. Mahoney (Myra McKinney), raids the Hyland ranch and kills the owner. Steve Armitage (Charles Starrett) arrives and lends his assistance to Mary Ellen Hyland (Gail Davis), the rancher's daughter, and her young brother Tod (Tommy Ivo). Steve learns that his old pal, Smiley (Smiley Burnette), is working for the Mahoney gang without realizing they are outlaws and killers, and also finds out the Mahoneys are anxious to grab the valley land because of an underground river running beneath it.