Bleak Night

High school student Gi Tae takes his own life. His father feels a heavy sense of guilt and confusion over his son's death. The father was largely absent from Gi Tae's life. He decides to search for answers to his son's death. Gi Tae's father finds a picture in his son's desk drawer. The picture consists of Gi Tae and high school friends Dong Yoon and Hee Joon. Gi Tae's father is able to locate Hee Joon. The high school boy is evasive with answers, explaining he transferred to another school prior to Gi Tae's death. Hee Joon also mentions that Gi Tae's best friend was Dong Hoon. Gi Tae's father thinks back to his son's funeral and recalls that Dong Hoon never showed up at his best friend's funeral. Gi Tae's father then asks Hee Joon to find Dong Yoon, who has now dropped out of school completely. What happened to these three former best friends?