Jeong Da-eun and Yun Jong-u are the sole known survivors of two killing rampages perpetrated by a mysterious gang. Many years later, an investigation into a possible copycat crime forces them to revisit their trauma. Da-eun becomes determined once again to find her boyfriend, Shim Gyuheon, who went missing after the murders they witnessed. Meanwhile, Jong-u is plagued by the ghost of "Room 203," his former neighbor who came within inches of killing him. His sick brother Jongseok was his only reason for living, but with Jongseok now dead, will he help Da-eun? And will Gyuheon ever be found?