Time between Dog and Wolf

Lee Soo-hyun (Lee goon-gi) becomes an NIS agent after he witnesses the brutal murder of his mother at the hands of a notorious gangster named Mao.  He decides to avenge her death and becomes part of an operation, only for it to fail at the last minute due to his childhood trauma and his inability to follow orders. 

Shortly afterwards, Soo-hyun is fired from the NIS, but is offered a lifeline by NIS Chief Jung Hak-soo (Kim Kap-soo) to work as an undercover agent called Kay, with his first mission being to infiltrate the gang.

The time between dog and wolf, based on the French expression L’heure entre chien et loup is commonly used to describe dusk or twilight.  It may also refer to a time when things cannot be easily distinguished from each other.