Benny Bear, Ruby Rabbit and Felix Fox discover clues suggesting that an old hilltop fort contains a forgotten tunnel hidden beneath one of its towers. By comparing the fort model with the visitor map, investigating sounds and airflow, and involving the museum guide instead of exploring the hidden passage themselves, they safely uncover the historic route. A fascinating mystery adventure that teaches observation, connecting evidence, logical reasoning, historical exploration, and the importance of putting safety first when investigating unfamiliar places.