A letter from an elderly aunt brings the Small family to Britain. There the children discover that Dame Hesselford, their mother's great aunt is being haunted. They decide to find out what is happening and why someone is trying to force the old lady from her home. Their search brings them into the mystery of the lost treasure of Queen Boadicea, a British queen who fought against the Romans. Their attempt at solving the mystery brings them into an underground passage and when their way to escape is blocked, it is up to the ingenuity of the children to find a way out.