The Longest Promise

The story takes place on a fantasy continent called Yunhuang (云荒), home to the Kongsang people (空桑), the merfolk, and the ice clan with its ten shamans. Beijing Daily notes that the world draws on the Shan Hai Jing (山海经, "Classic of Mountains and Seas").

Shi Ying (时影, Xiao Zhan, 肖战) is the crown prince of Kongsang. A favored consort frames him, and his mother, from the White clan, stages his death to save him. He is taken in by the high priest and withdraws to Jiuyi Mountain (九嶷山) to train as a priest, waiting for a chance to return.

Zhu Yan (朱颜, Ren Min, 任敏) is the only daughter of the Red King and a princess of the Chi clan (赤族) in the western wilds. Lively and fiercely loyal to her friends, she seeks out Shi Ying to learn magic so she can defend her land, and he takes her as his student. The Wenhui review adds that he knows from the start she is his ming jie (命劫, "fated ordeal"), yet never acts against her.

The series is adapted from Zhu Yan (朱颜), full title Mirror: Zhu Yan (镜·朱颜), the prequel to the Mirror (镜) fantasy novel series by Cang Yue (沧月), published in 2017. Jiang Jiajun (蒋家骏) directed and Wu Yingying (吴迎盈) wrote the script. Filming began at Hengdian on March 29, 2021 and wrapped in Yinchuan on August 4, with the desert and oasis scenes shot in Alxa League, Inner Mongolia.

Director Jiang told the press the team saw in the novel a pursuit of justice, equality and young people using what they have learned to protect others, and wanted viewers to take away more than a love story: a sense of duty to family and country, and willingness to put the self aside for others.

Ren Min said in an interview with Beijing News that this was her first fantasy role. She described Zhu Yan as a "little sun" who lights up those around her.