In the wake of Frank Best's dismissal, Inspector Jarvis promotes a reluctant Oliver to Acting Staff Sergeant. Today is also the day 15 Division gets a new rookie, Duncan Moore—an infuriating, eager, and connected millennial. Andy is up for the challenge and convinces Oliver to make her Duncan's Training Officer. But she may have acted too quickly… When Andy and Duncan end up as the first responders on a messy downtown homicide at a pawnshop, Andy has to try to show Duncan the ropes, while preventing him from compromising the case, himself, and his fellow officers.

Andy worries that her rookie, Duncan, may not have what it takes to succeed on the job. But her fears get pushed aside when they discover a man found shot in a stairwell at a dodgy apartment building - leading to the discovery of an executed gang leader in a nearby apartment. Now, it's all hands on deck at 15 Division to stop a potential gang war from breaking out. Andy and Duncan find themselves alone with the one witness who can tie the killers to the crime, and in a life-and-death moment with a 12-year old witness, Andy may regret giving her rookie so many chances. Meanwhile, Gail follows up on another victim of the tenement shootings - a woman that took a stray bullet in the laundry room - only to make a harrowing discovery that will shake Gail to her core.