On Sunday night, the case will finally be closed on CSI.

After 15 seasons, 337 episodes, three spin-offs and a handful of cast changes through the years, CBS is finally letting go of the show that reinvigorated its network. It's difficult to understate the influence of the show, which has grown to become the largest TV franchise in the world. You need look no further than all the imitators made in the show's likeness that have come and gone, both on CSI's home of CBS and other broadcast networks. But back in 2000, no one would have predicted that a show about the science of crime-solving would have such an impact.

The journey began in Las Vegas, where writer Anthony Zuiker was making $8-an-hour driving a tram. His moment of inspiration came while watching an episode of The New Detectives on the Discovery Channel about an Oakland Raiders cheerleader who had been murdered. "What really set it in motion for me was a long, long hair follicle with a tag cell that was caught in the headrest of the passenger side of the jeep," Zuiker tells TVGuide.com. "That signified, according to the narrator, that the hair was yanked out in a struggle. I thought, 'Wow, all of that information is connoted by a single hair follicle!' That really cemented for me the mantra of our series, which was the body is the perfect specimen. I felt like I could do a cop show that went below the tape for a forensic perspective with the sexy backdrop of Las Vegas, and the idea of CSI at that point was born."

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Zuiker's pitch caught the attention of Jerry Bruckheimer, who set up some meetings. While several networks, most notably ABC, passed on the show, CBS executive Nina Tassler decided to give it a shot. "CBS was just starting to go younger with Survivor," Zuiker says. "They [were trying] to do things a little differently and take some bigger swings. Nina Tassler told me in the most iconic words, 'If you write me a great script, I'll go upstairs and fight for it and get this on the air.' I wrote the script in three days, she kept her word and fought like hell during the upfronts." (Tassler, who recently announced plans to step down as chairman of CBS entertainment, was unavailable to comment for this story.)

But in some ways the show was still an afterthought. CBS slotted CSI on Friday nights at 9/8c, following a buzzy remake of the 1960s TV series The Fugitive. "We were the last thing put on the schedule," says William Petersen, who starred as Gil Grissom for the show's first nine seasons. "They called me the night before the upfronts and told me I had to get to New York! Of course, nobody knew [what would happen]. I think CBS just wanted us to hold some of The Fugitive's audience."

But CSI did much more than that. The show outperformed its lead-in, which lasted only one season, and was eventually moved to Thursday nights alongsideSurvivor, where it ran for 12 years. "I have to admit, it did take us all by surprise when it premiered with such a high number and exploded onto the scene," says CBS Television Studios President David Stapf, who at the time was the head of CBS' current programming department. "It was tremendously gratifying because a lot of work had gone into it, but you very rarely see a show that just sort of explodes. In hindsight, it's easy to go, 'Well, of course it did! It had three phenomenal showrunners, it had these incredible actors, it was unveiling a world that people didn't really know existed in criminal forensics and sort of pulled back the curtain on what these heroic crime fighters do.' But again, all of that is in hindsight."

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Although he admits he never could have predicted the global success the show has since found, Zuiker did have confidence in the format. "Anthony, from the very beginning, was talking about getting to 100 episodes when we were on Episode 2," says Carol Mendelsohn, who, along with Ann Donahue, was partnered with TV rookie Zuiker to run the show. "I really liked it, but I didn't know we were going to be talking about it 15 seasons later. There are some shows that come at a perfect time and in a perfect package. Forensics' time had come and it was a great way to do a cop show. ... Nobody knew how to note us because nobody understood forensics. We were on episode 9 or 10 and already kicking butt ratings-wise before anybody really paid a lot of attention to us. CBS was always behind the show, but we didn't get noted to death because the show was so different as conceived. We got to fly by the seat of our pants and be so creative."



The show also had an incredibly unique visual style that was brought to life by director Danny Cannon. From the way the show was lit and colored to its visual re-creations of crime scenes to the way the show tracked bullets through victims' bodies, it looked like nothing that had come before it. "It was huge," Zuiker says of the contribution made by the direction of the show. "There's nothing new under the sun, but it was new under the sun on television. Was I the first to do snapshots of body? No, Three Kings was - that's where I got the idea. Was I the first to do fractured time and flashbacks? No, I had a lot of influence from a movie called Run Lola Run. We borrowed a lot of styles, we added our own personal flavor and the cinematic way we make television until our show had a unified voice and a style. And we tried to be a nickel better every day. That's pretty much what we've done and it's worked."

But if you ask anyone involved with the creation of the show why it really worked with viewers, you will always get the same answer: Gil Grissom. "With no disrespect to any other actor who has ever been on our show, there is just no one else who can walk up and do what [Petersen] does on our show," Zuiker says "There's not many people who can be engaging with a look or draw you in by picking up a piece of evidence. It's really a piece of magic, and it's a masterful quality that you can't cast. He's the glue to the franchise by the mere presence of curiosity. That's something that isn't on the balance sheet in a lot of people's minds, but it really is everything to the success of our show."

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For his part, Petersen believes the show connected with audiences not because of any one performance, but because of what his character and his colleagues represented. "At the time the show came along in 2000, what happened that fall was the recount in Florida," he recalls. "There was a vagueness — people had a lot of doubts and they didn't know where to turn. Nobody was sure that anybody was telling them the truth. And I think Grissom, because he had eliminated his social personality, all he was interested in was the truth. The truth for him could be so simple and so specific that he'd pick up a toenail and say, 'I have the whole story. I know the truth.' He believed that there was no gray to this. What is, is. So, with the recount, no one knew what the hell was going on, but the guys on CSI knew what was going on. I think people tuned in because they wanted to see somebody know something."

Understanding that the spirit of Grissom has always loomed over the show even long after Petersen left (he was replaced by Laurence Fishburne for a few seasons and Ted Danson now leads the team), Sunday's two-hour series wrap-up (9/8c, CBS) wisely brings Petersen, as well as original cast members Marg Helgenbergerand Paul Guilfoyle, back to Vegas to solve one last crime. "It was like sliding back into the seat of your car," Petersen says. "It was great to be home. We've seen each other through the years, but it was nice to go back and act together as these characters because we like each other's characters. I like Catherine and Sara and Ecklie and Brass. I like them as characters, and I like Grissom. It was nice to be him again and it was nice to be him with them. It was nostalgic."

Zuiker is staying pretty mum on the details of the finale, other than acknowledging that there is an explosion that "paralyzes Las Vegas" and hinting that the episode will deal with the resolution of Grissom's relationships with both Lady Heather (Melinda Clarke) and fellow CSI Sara Sidle (Jorja Fox). In some ways, that feels ironic because of the show's insistence early on, partially at Petersen's behest, not to go home with its characters. But Zuiker says the TV landscape has changed, as has the audience's desire for character.

"Our quote back in the day was, 'We do character in dropperfuls,'" Zuiker says with a laugh. "The funny joke of it all is the one thing that wasn't scripted that really turned our series upside down was when Sara Sidle wiped a piece of chalk from drywall off Gil Grissom's face. The second she did that, it was the start of something that got people really interested. We will certainly explore the relationship between Grissom and Sara that started with that conversation and quite possibly build an entire finale around that."

But most importantly, Zuiker, Petersen and everyone involved are just thrilled to have the opportunity to say goodbye to the fans that made the show a success. "I was tempted to write the finale of the series as the pilot for the reboot," Zuiker jokes. "But the intent of the ending is to give closure to our characters and our fans. It's not designed to raise questions of a reboot. The focus was to put closure on the episode, and that's what we've done." Adds Petersen: "I feel great about it. We've done everything we could and hopefully we've done it as well as we could. I think we left it all out on the field, and hopefully the fans can forgive us for moving on."

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Whether the show ever gets a reboot or not, the show will linger for years to come. Not only is the fourth spin-off in the franchise, CSI: Cyber, still airing, but the original show, as well as the Miami and New York spin-offs still air in syndication. "It's an annuity; it doesn't go away," Stapf says. "Twenty years from now, CSI is still going to be sold globally as well as domestically. As a studio, we're still selling I Love Lucy, and CSI is that. It will be here 50 years from now. ... It transformed the network, and it transformed the studio. It gave us an identity and a brand and a strength in the global marketplace that allowed people to recognize that we do this type of show very well."

But the show will have other legacies as well. "I remember before the first season began, Billy said he didn't care if the show was on and off after 13 episodes, he wanted the show to make a difference, to be important," Mendelsohn says. "I don't think any of us back then could see that someone would someday coin the term 'the CSI effect.' But what the show has done is inform us that science has its place in the criminal justice system. I know a lot more stuff now thanks to Grissom and these CSIs than I did before I started the show, and I think that's a wonderful legacy." Adds Petersen: "They helped to find the truth, whatever the truth could be. CSI was smart and it was cool and it was fun. If you can be those things, you've pretty much got life by the tail."

And while the show will always represent the American Dream for a Las vegas tram driver-turned-TV writer, Zuiker is most proud of the mark the show will leave behind on the television business at large. "CSI was the one that really cemented the procedural in our business," he says. "For the most part, we've sent a good message, which is: If you commit a crime, here's how they're going to get you. And also: On the worst of day of your life, there are people out there who can scrutinize evidence and bring specific closure to a case and bring peace of mind to the survivor or victim's family. That is a universal mantra that translates all around the world in every language, and that's the one thing we definitely got right."

CSI's two-hour series finale airs Sunday at 9/8c on CBS. Will you be sad to see the show go?