Tom and his sister, Ruth, are home from school for a holiday. They are joined by their American friends, the Wheatley twins, Jane and Boyd. The four have already planned to explore Orleigh Cave, which has a local reputation of being haunted. But they are searching for a lost treasure of the Romans which they believe to lie somewhere in the cave. After being trapped in the cave by a rock fall, they travel deeper into the cave and stumble upon and into a secret world beneath the earth which is inhabited by descendants of the very Romans whose treasure they have been seeking. These people, with their debased Latin and their partly archaic and partly modern technology, guard their secret and their habitat rigorously from the upper earth. Fortunately the children make contact with a contemporary who has long been a prisoner and who has the aid of a "native" girl. But even with their new friends’ help, an escape attempt will put them in grave danger with only a chance of gaining freedom.