Fourteen-year-old Cassy Holt has just moved with her mom and horse to their new home in the Sierra Nevadas when she meets Jonathan Lane, a high school sophomore, fellow horse owner, and detective with a passion for exploring. But one night when Cassy hears the voice of Stardust, a mysterious girl who appears to be in trouble and asks her to find Nanny's cabin, Cassy has no idea she is about to be propelled on an adventure like no other. The next day, Cassy and Jonathan decide to ride their horses-Brave Horse and Moody Blue-to the forbidden valley to hopefully locate the cabin. After they learn a girl their age is missing, they gallop toward the abandoned cabin to seek answers. When they unearth Stardust's diary inside the cabin, they head toward home where they encounter someone seemingly on a mission to stop them. After they read Stardust's diary entries, Cassy and Jonathan soon realize that it is even more important to track her down, before it is too late. Will they find her before time runs out? In this exciting young-adult story, two high school students embark with their horses on a dangerous quest to uncover the mystery behind a local girl's disappearance.