His friendship with Mac Taylor is tested when Taylor uncovers evidence that one of Flack's men, Dean Truby, is a dirty cop and asks Don to turn over his memo notebook to prove it. Though Flack eventually capitulates, their relationship is a bit tense for a some time afterward (episode 3.08, "Consequences"; episode 3.10, "Sweet 16"). Flack defends Taylor to , who is investigating Sheldon Hawkes, the prime suspect in a robbery-homicide, after Taylor locks himself in the interrogation room with Sheldon. Flack affirms that Gerrard would do the same for him (episode 3.11, "Raising Shane").