When Uncle John forbids Mandie from joining his dangerous quest to keep an old promise, Mandie, her newfound mother, and her friends embark on a harrowing journey by train to protect Mandie's stubborn uncle. Grand adventure ensues as they scramble to outwit evil conspirators and dare to face the secrets of Lost Antler Cave. However, Mandie's most difficult challenge arises when she realized that success will land her face to face with the one person she never wanted to meet.