The unit receives a call about a shooting in the subway in which a woman named Beth McGuire was killed and her sister Angie was left injured. McKenzie notes that the bullets are the same as the ones that killed her brother Patrick who was shot while on duty trying to prevent a robbery. McKenzie's father, a veteran cop, puts the pressure on her to find Patrick's killer. Diaco matches the timestamp on subway surveillance with the metro card purchase time to determine that George Riddle was the buyer of the card used by the shooter. They question Riddle who suspects his employee Ronan Burke stole his wallet and identifies him in the surveillance photo. They bring Ronan into custody and a background search uncovers his ties to Lehane's Tavern, the bar of an old Irish mob with a long-standing beef against the McGuire family. They search his residence and find the gun with same ammo that killed Patrick, but records show Ronan wasn't in the country when he was murdered. Further digging into Angie's records reveals she was admitted to a hospital for sexual assault around the same time Patrick was killed two years ago. McKenzie detours from protocol and takes Angie to the alley where Patrick died, inciting her to admit that Patrick stepped in to stop Devlin Lehane and his accomplice as they were raping her. She kept quiet for fear Devlin would come back to kill her if she told the police. Clark stops McKenzie from going after Devlin on her own, and they are able to bring him in on health code violations for his bar instead. Owen sets Devlin up to feel paranoid that Ronan would make a deal with the police and they bug the prison cells to record the two men discussing the murder weapon as well as a safe located in the bar. They are able to arrest Devlin after finding evidence of his past crimes including Angie's earring and Patrick's watch in the safe. McKenzie and Ed rejoice about getting closure on Patrick's murder. Owen has reservations about retiring in two years.