person a is the heir to the throne of their slowly dying kingdom. with famine, debt, and a distant war raging, the kingdom's last option is to marry the heir off in order to save the realm.
person c is the heir to the throne of their large, powerful, wealthy, and fortunately neighboring kingdom. as their responsibility, person c's and person a's family decide on a betrothal to preserve and combine the kingdoms.
once person c and their court arrives, person a's attention fails to stay on their betrothed, becoming infatuated with one of person c's courtiers instead.
person b was a lady/valet of person c, only coming along to help their future ruler in anyway they could and were supposed to. when person a begins to pursue person b, they panic—a scandal like that could be the end of everything for them... hopefully, person c never finds out.