Chevalier Gaston de Chanley and Helene de Chaverny

Chevalier Gaston de Chanley loves Helene de Chaverny, raised in a convent as an orphan but in reality the daughter of somebody very highly placed in early 18th century France.

They long to be married, but Gaston has sworn an oath to a cabal of plotting Bretons to murder their enemy, the prince regent Philippe Duc D'Orleans. (Goodreads)