Every year, the team at Windy Creek Stables packs up the trailer and makes the trip to the famous Kentucky Horse Park for the best and biggest horse show of the year. Usually, the competition is Harper's favorite thing in the world, and she can't wait to show off for her new best friend, Presley, who is competing for the first time. This year, though, she's more than a little nervous. It's her first time competing against her old barn, Foxcroft Stables, and she isn't looking forward to seeing the mean girls who still ride there. Her anxiety only increases when she gets a midnight call that Isolde, her beautiful grey Andalusian mare, is sick. Luckily, Isolde recovers, but when another Windy Creek Stables horse starts to show signs of illness, Harper and Presley know something is up. One sick horse is just a sick horse, but two sick horses are a mystery. The girls must don their detective hats and figure out who's sabotaging the stables before anything else happens . . . and ace their competition as well, of course!