Jack and Annie follow a Jackrabbit to their magical tree house to find Morgan their Master Librarian guide, who is sending them to the Wild West to help solve a riddle, which is: Out of the blue my lonely voice calls out to you. What am I? After receiving their materials, including a research book about the Wild West in the 1800s and their mission, they are whirled away in their magic tree house. Once the land, they realize that they are in a ghost town called Rattlesnake Flats in Texas. They explore the town with its spooky, deserted buildings in search of the answer to the riddle. They hear a group of cowboys coming so they hide in barrels. While hidden, they overhear that the group are horse thieves who have separated a mother mare and her colt because the colt couldn’t keep up. After the group leaves, they find the colt that was left behind and a man named Slim and offer to help him get his horses back so the colt can be reunited with his mother. They go on an adventure and help Slim rescue the mustangs. Towards the end, they offer him the advice that he should give up herding and selling mustangs and write a book, like he originally came to the west to do. They travel back to modern times in their tree house and when they return, realize that Slim had written the book that they were using for research along their way. Even more amazing, Slim had dedicated the book to them! Oh, and Jack and Annie's riddle's answer was Echo.