I saw the killer, and the killer saw me.

Sang-hoon, an average salary man lives in an apartment complex with his wife and child at the foot of the mountain.
One night, he witnesses a crime scene through his living room window, and the killer also notices that he is being watched.
Sang-hoon, despite the dilemma, decides to hide and neglect the fact that he had seen someone get murdered, but had done nothing.
The killer, however, starts to search the apartment to find and eliminates his only witness of that night.