The cast of CSI: Crime Scene Investigation, past and present, gathered for a mini reunion in Beverly Hills on Wednesday before the show's series finale later this month.

"We premiered with not a lot of fanfare on a Friday night and 19 million people showed up to watch it," Marg Helgenberger told ET at the Paley Center for Media of CSI's 2000 debut.

WATCH: Marg Helgenberger on Returning for 'CSI' Farewell

Helgenberger is returning for the two-hour farewell after leaving the show in season 12. William Petersen will also be back after he parted ways in the show's ninth season.

"It's great," Petersen said about returning for the grand finale. "I've been gone for seven years. It was like getting into your old car again."

But saying goodbye to the groundbreaking franchise will be bittersweet for the CSI stars.

"You know what it boils down to? People that you go to work with every day," Ted Danson said. "You're going to miss them."

The show is ending its run after 15 seasons and launching numerous spinoffs, including CSI: NY, CSI: Miami and CSI: Cyber, which Danson will join as a regular.

MORE: 'CSI' Is Officially Ending After 15 Years

CSI's spectacular send-off airs Sunday, Sept. 27 on CBS.