Oppenheim was famous for his hundreds of spy and espionage novels. This is not one of them.
Duke Henry Chatfield with his family, and family lawyer Sir Stephen are riding through central Italy when the car breaks down in Pellini, where, 20 years earlier, the Dukes brother had a mistress and illegitimate child. Dukes daughter Monica is intrigued by a young Englishman named Francis taking his vows before disappearing into a monastery.
Three years later, the family lawyer discovers that Francis is in fact the legitimate Duke, and encourages him to return to England.