Busted! Season 1 Episode 8

A man claiming to be K’s old friend challenges the detectives to solve a 20-year-old case through a series of tests in order to measure their skills in criminal investigation. Each test has a different goal, and the first detective to complete a test is rewarded with a large sum of money. The man also promises to provide the passcode for the box that the detectives had obtained in Jeju Island, which contains valuable information about Project D. Outside the mission, Se-jeong is seen leaving flowers for M at a columbarium.