I got thoroughly lost in this one, and not the sort of lost that is enjoyable.
Published in 1925, this is one of the early ones of Christie's stories and includes political intrigue, jewel theft, blackmail, and the odd obligatory murder.
There are several issues with the plot, which is so thick that it took me a few rereads to figure out who is who and what is what. It didn't help that the main characters were from the fictitious country of Herzoslovakia - a Balkan state whose political history closely resembles that of Russia: The ruling monarch and his family having been murdered by the revolutionary forces.
Anyways, if the political meanderings don't put you off, the somewhat chauvinist characters and distinct xenophobia might just do it.
Not one of Agatha's finest.