After more than a quarter century as a Boston cop, and following a year filled with personal difficulties, Robert Twitch Twitchell has gotten a call: He's being promoted to Sergeant. Now, with just a week left as a detective on the gang unit, he's focused on bringing in one last criminal, a gang member wanted for assault with a firearm. Leading the search for the suspect, though, will require every bit of wisdom and experience he's acquired on the job. Meanwhile, on the night gang unit, it's a very different kind of week for Officer Myles Lawton, as the anniversary of the murder of his father, a former cop himself, is approaching. The emotions are complicated by a frustrating case in Roxbury, where a shooter got away, and no one seems to know anything about him.