Two men kidnapped a young man, a college student who spent his night in a bar with his friends. Unfortunately, there was another person who witnessed their action. Few hours had passed and the young man's body was now in a humid and dark place which seemingly was his kidnapper's hideout. Knowing that there was a witness, the kidnappers tried to find where this witness lived, attempting to kill him to dismiss any possible evidence he might know.

Meanwhile... in the other part of Seoul—

An annonymous letter came to an apartmen where a well-known detective lived with his best friend, a forensic doctor. The letter told them a very mysterious sign— it only said "21-3, die". Being curious of what had happened in the city, the forensic doctor—Kim Key—who received the letter at the first time, decided to find out. But sadly, it sent him to a very troublesome matter he himself could not solve.

Will the detective—Kim Jonghyun—helped his best friend to get out from this mess? Will he solve what is behind this kidnapping action? The mystery just... begins.