Flack is a homicide detective with the NYPD, known for both his dry wit and his determination to hunt down suspects, even to the point of chasing them down alleys and over rooftops. He was mentored by another cop, Gavin Moran, whom he is later forced to arrest after it comes to light that Moran concealed evidence during the course of an investigation (episode 1.17, "The Fall"). He is at least a second-generation cop; Moran mentions having worked with Flack's father as well. He is both a colleague and a friend to the members of the New York City Crime Lab. After Stella Bonasera is attacked in her apartment by her unstable boyfriend, Flack gently helps her through the aftermath when he comes to the hospital to take her statement (episode 2.21, "All Access").