This summer, nothing’s safe at Green Spring—not even the camp itself… Elsa, a friend of Nancy and George’s and a counselor at Green Spring Pony Club’s summer camp, invites the girls for a ride one afternoon. Along the way, Elsa gushes about how a team of campers will compete in a regional pony club rally. If they win, they’ll go to the national competition! But Elsa’s excitement quickly fades when Nancy’s horse falls into a ditch, and it’s clearly a case of sabotage. This prompts Elsa to tell Nancy about some sinister happenings on the camp’s grounds. Is someone trying to hurt the campers—or the camp? Disguised as a counselor, Nancy tries to figure out who's behind the vicious accidents. And as they become more devastating, Nancy realizes she needs to move quickly. Will her sleuthing skills be enough to keep this camp’s horses and their riders on track?