Mary, fourteen, and John, almost sixteen, are excited to go on a Ghost Alive Tour at the Warwick Castle in England, but soon find themselves surrounded by blackness in a small enclosure. After much effort, they head down a long, dark tunnel in search of their tour group. After traveling back in time they become fast friends with Richard and Katherine who are the oldest children of the Earl and Countess of the castle. The four children spend days hunting for a secret passageway so Mary and John can find their parents. Among their many adventures, Mary and John join the men of Warwick Castle in a crusade to the London Towers to rescue the Earl from prison, but he doesn't want to leave. There's a little girl that cries every night and disturbs Mary in her sleep. Is she a ghost in trouble or is she trying to help?