Mac and NYPD Detective Don Flack share a deep mutual respect and friendship, despite their very different temperaments. This trust becomes strained when Mac discovers that one of Flack's detectives is dirty. Mac asks for Flack's memo book to determine which officer was the culprit. Despite Flack's unwillingness to believe that one of his men is corrupt, he eventually capitulates and turns over the notebook. Their relationship remains tense for several episodes until they confront the issue in episode 3.10 "Sweet 16". When Mac locks himself in the interrogation room to talk freely with wrongly-accused Sheldon Hawkes, Flack defends Mac to his Captain, saying that they'd do the same for each other (episode 3.11 "Raising Shane"). Flack and Mac argue again briefly when a serial killer is released from prison thanks to the aforementioned arrest of the detective, but they quickly put aside their differences to catch the murderer and put him away for good (episode 3.21 "Past Imperfect").