Basically the first season starts off with an illegitimate princess resulted from an one-night stand trying to claim inheritance. The show then moves on as a slice-of-life drama about that princess. There is an arc in the second season which features radicalisation. treason, liberation of a woman from political marriage, and of course hostage rescue. The third season (which I have never got around to watch) apparently features the persecution and exile of the Fang family.
The Emperor of China had a one night stand with a random woman in central eastern China. He gave that woman a painting as a payment. Unbeknownst to both of them, that woman got pregnant. The Pretender Princess (PP) decided to embark on an inheritance claim after the death of her mother. She decided to use that painting to prove her identity.
The PP decided to attract the Emperor's attention by intercepting the royal procession, which of course failed miserably. She got rescued by another girl (the Fake Princess, FP). FP said that she will help PP to reclaim her legacy. FP decided to deliver the painting by gatecrashing the Emperor's hunting range while the Emperor was hunting, and she ended up getting shot by an arrow from one of the aristocrat.
So for quite a while, the emperor thinks that FP is his true daughter. The FP managed to get PP into the Forbidden City, in which PP serves as a servant for FP.
So yes, a Chinese painting without one's face is a perfect way to authenticate yourself in front of a Chinese emperor.
Anyway, this show is about the life of PP, FP and their friends in the Forbidden City.
The second season features the moral dilemma between Islamic radicalisation vs human trafficking. Basically PP et.al. tried to help one of the Emperor's Uyghurs princess concubine return to Xinjiang. So PP et.al. got radicalised by the Uyghurs concubine's ex-boyfriend. This Uyghurs princess ended up in Beijing due to some sort of political marriage which is against her will. You can argue that she got trafficked into Beijing.
So yes, I find that arc interesting, I really thought PP et.al stood for justice when I was a child - they help the Uyghurs princess to escape political marriage. But now I think PP et.al were pretty much radicalised, and they basically committed treason against the Chinese Empire.
The third season apparently features persecution against the Fang family. The third season was released like 10 years after the first 2 seasons, and it got pretty low ratings. So I haven't watched it yet.
Oh and of course this TV series features a lot of Chinese traditional tortures.