So the general's daughter Bai Chu meets a boy Lin Yu when she is very young. They are sweet on each other, but lost track of one another after boy does not show up for the mutually agreed meeting. Later on, a similar encounter happens between them, but this time the young man is in a predicament - he serves one of royal princes as his double or Shadow (hence the title Substitute). The royal sons Qi and Chen fight for the title of the Crown Prince and Lin Yu's sentiment for Bai Chu and common history is used by his employer to get general's backing via marriage with Bai Chu. Except, the prince is sickly and really not interested in the girl, so he sends his double to act as the prince. Chu and Yu are in love, but the prince knows that he will be assassinated, and Yu will have to pay the ultimate price for having a face identical to the prince. But the prince gives him a choice - marry the girl he likes and become the next Crown Prince or die in prince's place on the wedding day. As Bai Chu interacts with both men, not aware of the truth, she is both puzzled and elated that her childhood sweetheart is so cold and yet marrying her.
The wedding day comes and not only a man wearing the wedding gown is killed by the ruthless villain prince Qi, but also Chu's parents. The question we ask ourselves is: who really died? The prince or his guard? and the consequences are dire as revenge is necessary. What will happen now? The girl goes to train as an assassin for a year in Ghost Valley (not sure why she could not stick with poisons which she already knew), but she sucks at it even though she is sent to gather the evidence to prove the evil deeds of prince Qi, who has been grounded since his brother's wedding by the emperor. Bai Chu knows that a man died in her arms on her wedding day, so she is bewildered by the news that her husband has been recuperating in the palace this past year because his wound festers from the poison. And then she tries to steal the evidence from the man who OMG! looks like her dead husband! Ta-da!

This is a familiar trope of fated relationship and ultimate sacrifice - I am not giving away the ending, so if you are afraid of pain and sorrow, you just have to wait until someone will give away the spoiler. Or watch and have an epiphany.
This title was not on MDL until yesterday and I actually had to add it. After it appeared on IQIYI a few weeks ago I watched it right away because I just wanted to see Liang Xue Feng in another lead role (loved him as Young Master Hua in My Journey to You and was tracking his acting ability in 3 other series this year). While not disappointed with his acting, I thought the script and characters were blah. PLEASE no more villains who laugh like they perform in high school production of Wicked. On the other hand, Wang Ze Xuan, the crazy female-like prince from Hard to Find or the obsessed prince from A Familiar Stranger, is also here! And darn ! he plays well. FL is not bad, I have not seen her in other productions. OST is waltzy, which always lends itself to emotional support.