While returning from New Mexico to the airport in their rented bus, the Hollister children stop to buy cowboy suits. Mr. Hollister, with an eye for items to sell in his store, The Trading Post, discovers the suits are designed by Marie Vega, the wife of an old college friend. Mr. Hollister contacts the Vegas and the whole Hollister family is invited to spend a few weeks on the Vega ranch. Meanwhile, the children have met Helen and Jack Moore, youngsters about Pete’s age, who are staying at a dude ranch not far from the Vegas’ and Mystery Mountain. Jack has a map that leads to the cave of the ancient doll makers, the long buried secret of the mountain. Complications set in, however, when the Hollisters and the Moores meet the boy Willie and his friend the mysterious Mesquite Mike. Not only does the map disappear, but word is around that a rustler is taking sheep from the ranchers in the area. Here is another exciting Hollister adventure. What happens to the children when they spend a night on the desert? Is there really a monster in Mystery Mountain? And who could the rustler be? All these questions are answered in this fast-moving story.