"The Line of Duty" is a Most Wanted Side Mission in Batman Arkham Knight. You must find the locations of 15 firefighters and rescue them. Completing this mission unlocks the "Death of Innocents" trophy or achievement.
Play through the main story to get access to all of their locations. Some of them aren't available until much later in the game. After rescuing the last firefighter you must free the captain and bring him back to GCPD to finish the quest.