Reviewed by Christian E. (age 9)
Frank and Joe Hardy try to find out what is happening at the mill. They find a secret door to get into the main part of the mill. You can find out what secret they find in the mill, if you read this book.
I really liked this book because you don't know what is going to happen next. I liked the pictures, they showed what was happening and where. I liked the characters. I liked it when they were being chased, because you didn't know if they were going to get caught.
I think that anyone who likes mysteries should read this book.