In Police University, from the moment Dong-man (Cha Tae-hyun) found out that Sun-oh (Jin Young) hacked into the gamblers' account for his father's medical expenses, he had it in for the latter.
So when Sun-oh appeared in front of him again in Police University Episode 2 for an interview to be enrolled in the academy, Dong-man could not take it lightly. In fact, he abused Sun-oh and asked him how he dare come for the interview.
When the others in the interview panel ask him if Dong-man had information they needed to know, say something like a criminal record, he doesn't reveal the truth.
Dong-man decides to torture Sun-oh enough to get him to sign up for the program at Police University
Dong-man informed the other interviewers that it was a personal issue between him and Sun-oh. So the others decided to admit Sun-oh for the initial training. It is here that Dong-man made it his mission to torture Sun-oh in all possible ways.
In Police University Episode 2, Dong-man appointed Sun-oh and Kang-hee (Krystal) as team leaders. Whenever one of his team members committed a mistake, the one to get punished was Sun-oh.
According to Dong-man, a leader takes responsibility for everyone in his team, so he explained in Police University Episode 2 that he was teaching them skills to lead a team. However, Sun-oh was being taught why he didn't belong at the Police University.
At one point, when Sun-oh is tired of how Dong-man has used one incident to judge his character, he tried to understand why the officer hated him so much. Despite agreeing to let him go without filing any charges, Dong-man is extremely mad at Sun-oh.
Initially, it seemed as if the officer was mad for personal reasons. It was Sun-oh who had interrupted their operation, which failed miserably. This was the reason why Dong-man had been transferred to the academy.
A man who worked solely on-field for 20+ years and hated being tied down to a desk would resent anyone who put him there. Or so the audience would have thought. However, it was revealed that Sun-oh had not only stolen for his father's medical expenses.
When he left home to join the Police University, his friend and brother Seong-beom had signed on to serve in the marines as part of the compulsory military service.
Before Seong-beom had left, Sun-oh gave him a passbook for an account that held a considerable amount of money, which could help his brother go to college. This money was also stolen by Sun-oh.
However, Seong-beom was not aware of what Sun-oh had done. However, the detective knew of this, so he believed that Sun-oh did not belong at the Police University.
When Sun-oh asked for a second opportunity, Dong-man believed that he did not deserve this chance because he had stolen for a second time. The latter also walked away from Sun-oh after verbally thrashing him and instructed him to sign himself out of the training program.
At this exact moment, his partner, who had tried to track Hacker Yoon, Dong-man's friend on the dark web, succeeded. He informed Dong-man of the shocking truth that Sun-oh was Hacker Yoon. He is the friend that Dong-man had been very proud of.
Now, the question is if Dong-man will reconsider his stance about Sun-oh and give him a second chance after Police University Episode 2.