I know a bit about Bletchley Park and the Enigma machine and Britains battle to break the German's code during World War II. That was the most interesting part. It was also the part that wasn't really the focus.
Most of the novel deals with Lizzie desperate to find out the truth about her mother, Willa, who disappeared in Poland at the time of the German invasion. She and her brother, Jacob, work to uncover the clues left behind while holding down jobs at Bletchley Park.
For a book directed at tweenagers, it was very cute and readable. I wouldn't necessarily recommend it to an adult. There are much better books on the topic for that.
But it was an enjoyable, quick read.
