Catherine says, "Doc, you were here back then: tell me why this one's different." David can't: "Must be intimately familiar with the minutia of the prior cases. What's identical: cause of death, strangulation. Bruising on her wrists. Plastic zip-tie. Navy blue paint. There is one new element -- defensive wounds on the hands. Whatever she grabbed on to shattered and caused these lacerations." We see the glass embedded in Debbie's palms, and Gil asks, "Was that before or after she got the paint on her hands?" David tells him it was after, noting that one of the cuts sliced clear through the paint. Ouch. Gil notices that the paint is still wet, which is odd because Debbie's been dead for at least twelve hours. Gil asks, "What kind of paint doesn't dry in twelve hours, I wonder?"
Fortunately for the CSIs, the mtDNA tests peg Mathers as Charlene Roth's killer, but there are still two open questions: who killed the other women, and who killed poor Debbie Reston? We get two important clues: the blue paint used in Charlene Roth's death was different from the other deaths, and the ex-boyfriend of one of the victims, Cody Lewis, played the role of Red Herring in this evening's episode.
In the end, Mathers gets executed. Catherine watches, while Gil and Brass mutter uneasily over the idea of the Blue Paint Killer re-emerging:
The camera switches back to Brass drinking (my guess: whiskey) and saying, "He's going to kill again." Gil takes a drink and says, "Yeah. And all we've got is a partial fingerprint, and an M.O. that may lead us in the right direction." Brass comments, "Sometimes in this job, I'd rather be lucky than good. Maybe next time, we'll get lucky." Gil stares morosely at his drink and says, "I don't believe in luck. My only real purpose is to be smarter than the bad guys, to find the evidence that they didn't know they left behind, and make sense of it all. It makes me very uncomfortable to realize that this guy may be smarter than me."
Cut to Captain Exposition doing what he does best -- expositing in front of a whole group of people. He gives basically the same recap I just did, only without dramatic and contextually appropriate quotes. As Catherine explaining how the killer's MO is to apply never-drying blue paint to a hand rail and nab victims at a nearby fountain when they're washing their hands, we see Nicky inspecting a handrail while Sara prints a fountain. And then Brass is all, "No rest until we bring in this guy!" while Gil sits in a darkened room and ponders the killer.