You know that fancy new CSI contract that William Petersen inked last May? Turns out there was a ticking time bomb buried in the fine print: He'll be gone by midseason! And not "off too do a play" gone like he was back in 2006. No, this season's 10th episode will be his last as a full-time series regular.

"Billy is leaving," confirms the show's executive producer, Carol Mendelsohn. "But he will remain throughout the run of the series an executive producer. And he will, whenever CBS asks, come back. I don't think you've seen the last of Gil Grissom."

Neither does Petersen. In fact, in an exclusive interview conducted on July 11, the eight-year CSI vet seemed almost reluctant to characterize his departure as, um, a departure. Citing both his ongoing role as an executive producer and his intention to return on occasion as a (very special) guest star, he said, "I'm in a great place in terms of knowing that I'll be more free to make choices. And I'm responsible enough to not do it in a way that would hurt [the show].

"I want it to work for the writers, I want it to work for the cast, and, most importantly, I want it to work for the audience," he added. "I don't want them to abandon the show."

CBS has to be concerned about that, too. Although no one expects ratings to nosedive when Petersen leaves, the network's top-rated show is staring down its biggest challenge ever. His sorta swan song, coupled with Jorja Fox and Gary Dourdan's exits last season, brings to three the number of original cast members CSI will have lost in a year's time.

"This will be a year of transition for the show," concedes Mendehlson. "We always knew it was coming, and I'm glad it didn't come until now." At this point, she's confident that, like Law & Order and ER, CSI is a franchise that can weather such cast shifts. "There's never going to be another William Petersen, and there will never be another Jorja Fox," she hastens to add. "But we are not out to just replace those actors. We're out to add new characters to the show, and the show will change as a result. But that's a good thing." (For more on CSI's post-Grissom plans, click here.)

In the meantime, Mendelsohn is plotting one hell of a send-off for her leading man. Just as this is a year of transition for CSI, so too is it for Grissom. The events of last season's finale -- specifically the death of Dourdan's Warrick -- will push Grissom to the breaking point. "The easiest way to describe Grissom is 'in crisis,'" Petersen told me. "As a man. As a scientist. As a teacher. As a middle-aged person who has been very successful at what he's done... [he] wonders, 'What's the point?'

"It's all of a sudden becoming more difficult for him to do his job," he continued. "What had been a sort of fun job for him to do, because he loved the solving of the riddle, has become ever more taxing and difficult."

Helping Grissom through the crisis will be his true love, Sara, played by Fox, who is returning for multiple episodes, starting with the season premiere. While Petersen confirms that Sara will figure into Grissom's final arc, he stops just short of saying the couple will ride off into the desert together at the end of episode 10. "I wouldn't want to say exactly what we're going do -- I want people to watch, certainly," he chuckled. "But Sara is involved... It's often darkest just before the dawn."

And lest anyone read any hidden meaning into that statement, the actor reiterated, "For me, it's a really good situation. I don't want the audience to think it's not... [It isn't] like there's something going on, like 'Petersen's unhappy,' 'cause it's not true. It's quite the contrary. "

That explains why he's having trouble labeling his departure a... well, you know. "My biggest problem with leaving the show at any point isn't leaving Grissom -- I'm an actor, it's time to do other things," said the stage vet, whose first post-CSI gig will be a return to the theater -- and to Connor McPherson's play, A Dublin Carol -- this fall in his hometown of Chicago. "[The hard part will be saying goodbye] to the cast and crew. [So] I'm going to stay as involved as I'm allowed to be throughout the remainder of this show. Until they turn off all the lights."

* Current cast members Liz Vassey (lab rat Wendy) and David Berman (coroner David) have been upped to full-fledged series regulars. Wallace Langham's Hodges, meanwhile, will become more prominent.