Voight stands outside Gradishar's office at Internal Affairs as she packs her things. The falling out cost Gradishar her job, but she assures Voight she'll be there to watch him fall. Her IA replacement contacts Voight on his own - a very self-confident button-down named Stillwell. He pressures Voight on a familiar subject: Maurice. Take him down ASAP or suffer the consequences. Voight shrugs him off, but gets a major shock when he finds a new detective in the bullpen, ready for work. She introduces herself as Sumner; Stillwell sent her down. His blood boiling, Voight relegates her to paperwork downstairs.
Across town at a heroin flophouse, Nadia, the junkie that Lindsay tried to sober up, reaches out to her. She's finally ready to get clean and wants the detective's help. Lindsay offers up her apartment in lieu of an opening at a clinic and instructs Nadia not to leave.
A call sends Intelligence to a surgical series of executions at a secret Triad gambling ring in Chinatown, clearly a professional job. Off some unregistered plates found on the scene, the team hits up another undercover den and sends Halstead inside for a look-see. He ends up with a gun to his head - a cop's gun - and talks his way back outside without revealing anything. Back at District 21, Halstead ID's a couple men he noticed inside, and Voight recognizes one as his ex-partner, Jimmy Shi. Is this guy dirty? Voight doesn't think so, but they tail Jimmy and his crew - all toting serious firepower - to a shady club and just as it looks like another gambling shakedown, Voight orders a raid. But when Jimmy goes toe-to-toe with Voight, he explains that his operation is legit police work, 11 years in the making. Intelligence just sabotaged the last step - Jimmy was about to get names on everyone in the Triad.
Jimmy lays into Voight, livid about the mistake, until Halstead interrupts his rage with news of another robbery/murder, this one at a location Jimmy's unit visited before the botched score. Based on the crime scene and the time of the crime, Voight makes an astute observation - someone's impersonating Jimmy's team.
Meanwhile, Olinksy notes Ruzek's increased on-the-job swagger and makes a bold move - he orders Ruzek and Atwater to switch for the day, with Ruzek joining Burgess on patrol and Atwater catching the bump to Intelligence. Ruzek labors through a tame shift with Burgess, the only exception being a quick burglary call that almost ends with a knife to Ruzek's face. The experience of the day teaches Ruzek some serious humility, but something unexpected also occurs: although he never says it out loud, he and Burgess share some kind of connection. Or is it an attraction?
Atwater experiences the opposite as Intelligence ramps up its investigation and agrees to work with Jimmy's squad to set up a sting and lure the real criminals into a fake gambling den. The impersonators take the bait, but the operation goes south when they use an unexpected flash bomb that puts Atwater, Olinksy and Antonio out of commission. This spurs a chase into the heart of Chinatown where Jimmy crashes his SUV into the suspects' car to enable an arrest. Lindsay takes note as Voight comes to Jimmy's aid and helps him out of the totaled vehicle. There's history between these two...
Lindsay prods Antonio for an answer and he conjectures that Voight, Olinksy and Jimmy covered up the murder of a suspect accused of killing Olinksy's old partner. With that on her mind, Lindsay returns to her apartment - and a missing Nadia. A quick phone call solidifies it: she relapsed again.
A botched arrest forces Voight to team up with his ex-partner.