Hannay meets Lady Madrigal Fitzjames on the train and they discover that they are both going to the same wedding function.
However they get off the wrong stop and are mistaken for the married couple. They are taken to a smaller mansion, a stop before the happy couple go on their honeymoon. They are greeted by the staff. No one seems to twig that they are not the married couple.
In the mansion Lady Madrigal is presented by a family heirloom. A valuable diamond necklace which is then placed in a safe.
Hannay discovers later that night that necklace is missing with the staff trying to get away.
The necklace is there again in the morning with the staff. Hannay notices that the necklace was fake and the staff returned it as they want the genuine article. This time they turn nasty on the guests.
At times this was a ludicrous episode as you wonder where were the real married couple given that there was a greeting party waiting for them in the train station.
Lady Madrigal quickly got to be irritating and if she could recognise fake diamonds then she should had spotted them when she wore the necklace for the first time. Also bringing that newspaper with her was mighty convenient.
At least this one is a crime mystery concerning a gang of jewel thieves than espionage. Still Hannay gets to do some heroics.