Dawn and her fiancé Henry hire canoeist Ace to guide them to a lost valley in the Texas Trans-Pecos where Dawn's father disappeared twenty years earlier. Their search leads to a lush jungle world ruled by an evil medicine man who commands an ages-old stegosaurus to carry out his bidding. Will Dawn find her father? Or will she fall victim to the Sinister Secret of Stegosaurus Slope?A side trip in the Rhun-Ry'tbai cycle dedicated to the spirit of Edgar Rice Burroughs.