When an armed suspect is uncovered at a secured crime scene, Danny Messer's routine investigation quickly becomes a foot pursuit through the subway tunnels of New York City. Caught in the crosshairs, Danny opens fire, but upon investigation learns the man he shot is an undercover cop. Officers and witnesses believe Danny killed the wrong guy and it's up to Mac to prove otherwise. On the other side of town, the murder of a nanny leads Stella Bonasera into the startling world of those who care for the privileged children of the Upper Westside. She discovers the victim was trying to back out of a theft ring among the nannies, when one of them killed her.