Nirvana in Fire Episode 13

Since Minister He became sickly after hearing his son would be executed, Prince Yu decides to try to save his son. Meanwhile, Yujin and Jingrui come to visit Mei Changsu. Yujin brings in tangerines, but when Fei Liu turns them down, Mei suspects that the fruit was part of a shipment of smuggled gunpowder. When Prince Jing goes to visit his mother, Concubine Jing tells him about the herb that caused the Empress' illness; Prince Jing later relays the information to Mei. Mei realizes that it is Marquise Yan Que who smuggled the gunpowder, so that he can use it to kill the Emperor. It was also he who harmlessly poisoned his sister, the Empress, so that she would not be present when the gunpowder is set off. However, Mei successfully persuades him not to carry out the deed. (Source: Wikipedia)