Laurence Fishburne and William Petersen, CSI

They're the words no fan of CBS' top-rated procedural ever wanted (or expected) to hear from Grissom: "I'm going to leave CSI."

Yet that announcement comes within the first minutes of this week's episode, laying the foundation for William Petersen's exit as a series regular and in some part paving the way for a new arrival to the team: Ray Langston, a criminology professor/former research pathologist played by film vet Laurence Fishburne.

But before Fishburne hits your screen — and amid the undoubted cries of, "My Gil/Bill is not replaceable!" — just know these four things about the "transition of power" to come.

Fishburne Isn't Replacing Petersen
The man who led us into the Matrix is a commanding presence, to be sure, but he won't be commanding the Las Vegas CSI team. Yes, once the dust settles, Petersen will be gone and Fishburne will be in place. But Langston ultimately joins up as a CSI-Level 1 —a plebe, albeit a highly educated one. Instead, in this week's episode, Grissom crowns Catherine (Marg Helgenberger) as the new supervisor. And logically so.

Fishburne's Character Is Not a Serial Killer
At this summer's TCA press tour, the announcement of Fishburne's casting included some muddled allusion to his character possessing a DNA trait common to serial killers. (Cue a collective, "Wha?!" from us reporter folk, as well as probably a slew of geneticists.) That buzz was quickly "cleared up" in a subsequent conference call. "It was misreported a little bit," said executive producer Naren Shankar. "What we really want to play with in a dramatic sense is he's been able to examine his own genetic profile to an extent and find certain clusters of biological facts that he has in common or that are associated with aggressive and criminal behavior."

Grissom Isn't Leaving Just Yet
Although he announces his departure in the Dec. 11 episode — prompting one perturbed coworker to snark, "You gotta do what you gotta do. Have a nice life," while another introspectively tells him, "I knew before you knew" — Grissom's final appearance as a regular character isn't until Jan. 15.

Grissom Isn't Gone for Good
Petersen and CBS have said from the onset of this discussion that the actor and Gil will resurface on an intermittent, guest-star basis. Plus, Petersen is staying on board as an executive producer. Seriously, you didn't expect Grissom to stay away from the fascinating bugs forever, did you?