After an explosion at a meth lab leaves one person dead, Tom and Ray suspect a feuding couple. However when one of them turns up dead near police headquarters, they must refocus their investigation on someone out for revenge. Teresa begins to have feelings for Joel.
Ray and Fearless respond to silent alarm and find the cops who first responded to the alarm shot dead. They deduce that the robbers knew they were coming so they ambushed them. They wonder how they knew were coming that's when they discover that they got a call from their station, meaning someone there called to warn them. The Captain has the whole precinct placed on lock-down. Joel, who was off duty, drops by. The Captain asks him if he's willing to question everyone at the station, he reluctantly agrees. At the same time, Fearless and Ray try to find out who it is on the outside. Fearless gets a tip off that the gang leader who was on death row (who threatened to kill a cop unless his sentenced was commuted) was aided by another cop to grab the one he got, and that he's the one whom Fearless is looking for. Ray finds himself being summoned by McNorris who raises Ray's rumored graft again.
After a Russian mobster kills a man in a public restaurant, a little girl comes forward as the "only witness" to the crime. Fearless must decide whether to have the girl testify and risk her safety or have her keep quiet and allow the Russian mobster to go free, putting others in possible danger.
Joel, Fearless, Ray and Tom team up to investigate a group of cop killers, who threatened Joel's family. Robbery/Homicide detective Katherine Pierce is reassigned to assist. Meanwhile, David goes to rehab and Teresa thinks about her future.