After 15 years and countless cases, CSI closes the lab for good with a two-hour send-off.

THE MOST POPULAR, groundbreaking and influential whodunit in TV history has to go out with a bang, right? CSI: Crime Scene Investigation ends its 15-season run with “Immortality,” a two-hour episode about an explosive attack on the Eclipse, the posh Las Vegas casino co-owned by former CSI Catherine Willows (Marg Helgenberger). This news-making calamity has Catherine, now working for the FBI in Los Angeles, returning to Sin City to lead the forensics investigation. She’ll be joined by her old boss, Gil Grissom (William Petersen), who is back to help with the case because he, too, has a personal connection: One of the prime suspects is his former dominatrix love, Lady Heather (Melinda Clarke).

“It’s a privilege to be able to bring real closure to this show - something that rarely happens when a program gets canceled,” says creator Anthony Zuiker, who was making eight bucks an hour as a Vegas tram driver when he sold his CSI pilot to CBS. “Our finale is scary and thrilling and entertaining as hell, but it’s also a chance to thank the fans for their undying devotion,” Zuiker says. “We want them to feel good about what happens to this family of CSI characters when the show is over. To do that, we’ve pulled out all the stops.”

But first Zuiker took to social media to find out exactly what those loyal fans hoped to see resolved before CSI ends. No surprise there: They rapidly want divorcées Gil and Sara (Jorja Fox) to get back together. “Can we please everybody with this finale? Of course not,” Zuiker teases. “But will we please most of the people and give them the emotional experience they want? Absolutely.”

This last hurrah certainly had Helgenberger in a state. The Emmy-winning actress, who exited CSI in 2012, read the finale script while flying cross-country. “When I got to the last page and saw the words ‘End of series,’ I burst into tears,” she says. “The flight attendant was very concerned about me. I never would have anticipated such a reaction, but, obviously, this show means a great deal to me. I just had to come back and see it through to the end.”

CSI stunned us with its crafty crimes and state-of-the-art visuals of slo-mo bullets ripping through organs and chest cavities, but it also shook up the U.S. legal system. Don’t believe us? Google “the CSI effect”! The series spawned three spinoffs, too, including CSI: Cyber, which returns this fall. “We made science cool and sexy, and in our heyday, we were pulling nearly 30 million viewers a week,” Helgenberger says. “Who does that anymore?”