It's another crossover event with Chicago PD and Chicago Fire. 

The action kicks off on Chicago Fire with Joe Cruz going undercover at a neighboring firehouse to figure out which firefighter is stealing a lockbox key used in a string of robberies. 

Before the end of his shift, he oversees Suggs taking the key and handing it off to a car outside. 

Cruz confronts him, but Suggs promises he can make things right. He tells Voight everything, and shortly after, Intelligence finds Suggs murdered. 

A murder investigation is opened up, however, Ray and the CFD ask Voight to turn a blind eye to Suggs' involvement so his family can reap the benefits of his pension. 

Voight is determined to get to the bottom of the case, and when another burglary alarm is triggered in a high-rise, he sends his team out there. 

Halstead finds a young man in the apartment administering first-aid to a man who wasn't supposed to be in the apartment when they were robbing it. 

The kid tells Halstead that the man got spooked, fell and hit his head. 

Halstead takes over compressions and the kid gets away. 

However, the fact that the kids knew CPR and how to administer aid means that he had to have taken a class. 

Burgess realizes that Suggs used to volunteer at local high schools and before you know it, they have the 15-year-old, Aiden, identified. 

They scoop him up in his neighborhood, but during interrogations, Aiden tries to take the blame for the robberies and the murder.

Voight and Halstead won't allow that to happen, so they find his accomplice, Chris. 

They question the boys in the same spot Suggs was killed because no cameras and no cops are around meaning there's no confession on record. 

Both the boys come clean about their involvement in the robberies, but they also admit that Sugg was helping them buy back their freedom from a local gang. 

Kamen, the gang leader, said their freedom came with a $100 thousand dollar price tag, so Suggs was stealing the keys to help them pull off the heists. 

Voight knows he can't tarnish a Chicago firefighter, but he also doesn't have a compelling case to put Kamen away. 

Halstead asks if they either of the guys would go undercover and Aiden agrees since he blames himself for Suggs' murder. He tells the cops that he'd accidentally told Kamen about the key.