Maggie Hope fails out of the MI-5 boot camp due to her physical strength (and the lack thereof). However, just because she is not cut out to go overseas undercover, she is still capable and an asset to use on home soil. So she is given an assignment - tutor Princess Elizabeth in advance maths while really searching out for any saboteurs or German spies working in Windsor Castle.
This was an interesting and fast paced story once Maggie is at Windsor. The last book dealt with Irish rebels; this book deals with the British Fascist Party and the German spy network. Along with protecting the princesses and sniffing out the saboteurs, Maggie finds out more about her parents and the beginnings of love triangle form. And it turns out, Princess Elizabeth is a very astute student, putting Hope's lesson on cryptography into good use. Maggie is good at maths, not so much reading people - I hope with more experience, she is gets better at that much needed skill.