When Katie Cooper and her best friend Sarah Owens are falsely accused by a state trooper of vandalizing a car, they vow to solve the case. Katie and her friends conduct a thorough investigation only to find that the evidence points to several possible suspects, including the notorious Zorro Gang. As they work the case, they spend time in the woods of Mt. Pleasant, where they also solve the mystery of the Mountain Man! In this series of mysteries, Katie Cooper and her friends investigate some serious crimes. In the process, they meet some unusual people and learn a whole lot as they carry out their investigations.