Joel and Fearless enlist Ray and Tom in an covert op to track down the thieves who killed Wisnicki and Chandler when the detectives assigned to the case fail to go after John Donadoni, the man behind the murders; when tailing Donadoni leads to Sabrina Fithian, Joel and Fearless break into her apartment to wire it, and stumble into a surveillance led by Katherine Pierce, a Robbery/Homicide detective who's been after Sabrina, and knows that Sabrina and her partner Rachel are the cop killers they're after; after Donadoni threatens Kelly and Willy, Joel lets Donadoni know that Donadoni's mother can also find herself in danger; at Joel's urging, Katherine gets herself reassigned to their squad and gets Ron to assign them to her task force; Sabrina is captured after the team interrupts her latest burglary, but not before Sabrina wounds Katherine and then shoots a two-timing Rachel with Katherine's gun; Joel warns Donadoni that he's a marked man, no matter what happens to the case against S

David comes out of rehab and lands in the middle of the case against Sabrina and Donadoni, although not in the position he'd like at first; Ray and Katherine get bloodied on the witness stand at the preliminary hearing by Sabrina's attorney; David's struggle to stay sober is sorely tested when his father comes for a visit bearing extremely troubling news; when the diamonds stolen by Sabrina and Rachel can't be found, the task force puts their heads together with David to recover the gems, find the key to getting Sabrina to do a full flip on Donadoni, and put Donadoni behind bars.

Called to the police academy to investigate a murder, Joel and Fearless discover that the recruit who discovered the body is none other than Teresa; Ray and Tom work with Katherine to stop a ring of thieves posing as police officers.

Bank robbers attempting to flee the scene end up at a local shopping mall and take several hostages, including Teresa, whose ""shoot, don't shoot"" lessons at the academy come in handily at a critical moment.

When the body of a college student is found in a house under renovation, David joins forces with Katherine, Joel, and Fearless to determine which of the victim's frat brothers is responsible for his demise and burial in the wall of their former frat house twenty years earlier.

Wracked with guilt for not following his instincts on a domestic disturbance call when the woman involved is reported missing and presumed dead several hours later, Tom works with Katherine to uncover the whereabouts of the woman and the person who murdered the chief suspect in her disappearance.