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.