Marg Helgenberger on Returning for 'CSI' Farewell: 'One More for the Road'

The actress reveals to ET that William Petersen is working on the final story with creator Anthony Zuiker.

Fans of CSI will have one last chance to say goodbye to their favorite forensic team.

CBS made it official last week when it announced that the long-running crime procedural would end after 15 seasons with a two-hour send-off in the fall – but it was news that original stars William Petersen and Marg Helgenberger that had loyal viewers rejoicing!

caught up with Helgenberger during Monday’s CBS Summer Press Day at The London Hotel in West Hollywood to get the scoop on what fans can expect come September – and to reflect on the breakthrough series.

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CSI was one of the best experiences I've ever had career-wise. Talk about a phenomenon,” Helgenberger, who next stars as an anthropologist in CBS summer drama Under the Dome, told ET. “Besides its success and it being an innovative show in its day, it was an incredible group of people.”

Helgenberger, who departed CSI in season 12, reflected on the show’s legacy and why it was a no-brainer for her to reprise her role as Catherine Willows.

“The show was so unique when it came out and it spawned all the spinoffs and other [similar] series,” she said. “I just thought, ‘Why not come back and experience that one last time?’ One more for the road with this group of people.”

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She won’t be the only original castmember returning for CSI’s swan song. Petersen, who left after season nine, will also be back as Gil Grissom. Helgenberger revealed that Petersen will be contributing to the story as well.

“I'm eager to see what this script will be because Billy [William] Petersen is apparently working with Anthony Zuiker, who created the show, on the story,” Helgenberger said. “Billy's got great taste and he's a smart guy. I'm happy to get in the ring with him because it's been a long time.”

’s two-hour swan song airs Sunday, Sept. 27 on CBS.

ET was on the set with Marg Helgenberger in 2011 as she filmed her final episode as a regular on CSI. Relive the emotional moment below.