The same basic setup as Curse of the Golden Flower: Tail end of the Tang dynasty in the Chinese imperial court in the late 9th century.
Whereas Curse had the Empress as the figure the story revolves around, Banquet revolves around the Crown Prince, who has left the court to be am actor. He is dragged back into the imperial intrigue when he gets news that his father, the Emperor, has died from what can best be described as less than clear conditions.
Complicating the matter is that the Empress and him have a relationship, as well as his love for the daughter of a high official. The Emperor's brother has lain claim to the throne, and has decided to marry the Empress.
The Empress, meanwhile, is not happy that the Prince has another lover, is not happy with the new Emperor, and wants the prince for herself.
Complicating matters (as if they are not complicated enough) is that the father of the prince's par amour wants his son to be Emperor.

Whew...