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 Sabrina; David's still running an angle, even in rehab.