An assemblyman and an attorney were killed by Simon, the most notorious drug dealer on the West Coast. The killer holed up, so the CSI team has to track down him to put him in jail. Keppler convinced Catherine and Brass to approach this case in a different way, so-called 'reverse forensics', in which investigators manipulate a crime scene and arrest a fake suspect in an attempt to make the real suspect think he is off the hook and feel free to do something. Frankly, that's the way I've always thought about personally that the CSI team could try, at least once, in order to catch the suspect. (Probably, it would not have happened with Grissom in charge.)
When the other four members - Nick, Warrick, Sara, and Greg - noticed something fishy going on between Catherine and Keppler, they decided to work on the investigation by themselves. Especially, it seemed like Nick felt betrayed really bad. Anyway, the team managed to nail Simon, but Nick is still angry at the fact that he was deceived by someone he has always trusted.