I won't summarize the story, but if I were to do so, it is likely to sound like a fairly good story with a good measure of suspense and a nice twist. Yet, there is something underwhelming about the series.

To be fair, the basic story of a serial killer out on the loose is quite good. The actors have also done quite a good job overall. Shen Yu is probably the most complex character, and I appreciate the ambiguity in her character, which helps with the building of suspense.

On the other hand, the ambiguity in Shen Yu, which does not go away even at the end of the story, may not be handled very deftly. At times, she seems almost psychopathic, like how she smiles when she deliberately gets a woman (who has killed her husband after years of abuse) agitated. Perhaps because she a main character, there was a decision not to make the character tilt too much to the dark side eventually, but there is a need for more overall coherence even if there is ambiguity in the characterization.

Then there are also the peripheral cases that don't seem all that relevant to the main story about the serial killer. The case of the murder of a pet killer does not appear to have been resolved very fully. In the end, it seems to have been written in solely for the purpose of showcasing Xia Mu's sleuthing ability (something that is emphasized at the start but ultimately does not play such a huge role) and casting doubt on Shen Yu's character. This is an instance of how the details in the telling of an otherwise decent basic story lacks finesse.

Somehow, the story also contains distractions that border on the implausible and they don't even qualify as red herrings (which can sometimes be impressive if done properly). The way one of the characters is (almost) scapegoated by the serial killer stretches one's ability to suspend one's disbelief. The sheer number of coincidences, the arrangements that need to be made and the psychological tactics that need to be employed with the exact effects intended (when there is no way to be sure of their effectiveness) weaken the story as a whole.

The serial killer is supposed to be someone really intelligent, meticulous, resourceful and probably has plenty of free time on his hands to watch some of the characters. Then when the identity of the killer is revealed, it is a bit of a letdown. He seems delusional (though inconsistently so) and does not exude the aura of a killer who has been able to conceal his crimes for almost three decades. The extent of his cruelty (such as placing a clock in front of his victims so that they see the clock ticking as they die slowly), the chosen method of killing (using a mix of anesthesia and muscle relaxants) and how he obtains the material needed for the killings are all unaccounted for. (To make matters worse, what does get explained, like how he learnt to give injections, can be rather bizarre if you think about it.)

Finally, although there is supposed to be a lot of emotional intensity, I somehow don't feel it. I know the cop (Leng Xiao Bing) who misses a chance to shoot the serial killer is supposed to be guilt-ridden, and the actor does a good job portraying it, but the scenes don't really convince me of it. We don't really see the psychological depth behind his inaction; neither do we really see much of his struggles with guilt over the years. Xia Mu is supposed to be suffering from PTSD firstly due to the murder of his teacher (who is like a mother) and later due the fact that grandfather is killed, but we get too few glimpses of how attached he is to these two people in his life. (In fact, he doesn't even seem to have witnessed the grandfather being killed, but it triggered a relapse of the PTSD?)

There are elements of the story that can make for a relatively good sales pitch. A serial killer is on the loose. A cop wants to correct the error he has made as a rookie. A young man becomes a cop to nab to solve a cold case because his loved one is a victim. A doctor who may or may not be abetting or even instigating crimes. Unfortunately, the series makes one expect so much only to deliver so little.