When Uncle John forbids Mandie (Lexi Johnson) from joining his dangerous quest to keep an old promise, Mandie and her newfound Mother take matters into their own hands. Joined by friends, they embark on a harrowing journey by train to protect Mandie's stubborn Uncle John. Grand adventure ensues as they race to outwit evil conspirators and dare to face the secrets of Lost Antler Cave-Mine. However, Mandie's most difficult challenge arises when she realizes that success will land her face to face with the one person she never wanted to meet.