[Spoiler alert: The following article contains information about the upcoming “CSI” finale.]

After 15 years, "CSI: Crime Scene Investigation" will wrap up the series next month with a two-hour finale that will see the return of original cast members William Petersen, who played Gil Grissom, and Marg Helgenberger as Catherine Willows.

"What I can share is that it is going to be a crowd pleaser, and the true diehard fans will be satisfied," Helgenberger told FOX411 at the CBS Television Critics Association party on Tuesday. "There's a lot of story that will get wrapped up and a lot of fan favorite characters are brought back. I was satisfied.  I think fans will be, too.”

One of the returning characters will be Lady Heather (Melinda Clarke), who transformed throughout the series from a dominatrix to a therapist specializing in sexual counseling.

"For the true fans that watch that show, they know that character had a real impact on Grissom and then the whole Sara [Jorja Fox] thing happened," Helgenberger said. "Lady Heather weighs heavily in the story in terms of the crime that is committed and in terms of the Sara/Grissom relationship."

As for the rest of the whodunit, Helenberger is sworn to secrecy. She dished that every time she gets updated script pages there is a note included reminding her not to share the information. But she was able to reveal that Catherine, who left town to take a job with the FBI, is lured back when a federal crime is committed at one of the casinos she inherited from her father, Sam Braun (Scott Wilson).

"When Sam died, he had other family members but one of them is in jail, so I was the sole recipient of his legacy of all the casinos," she shared, recalling of how her character came to be a wealthy woman.

Helgenberger said when she left the show to pursue other projects, she didn't miss Catherine at all, but that changed when she put Catherine's suit back on and started reciting her lines.

"I thought, 'I love being back in this women's soul.' She was always direct and tough and passionate about what she did, and told it like it was. She loved her daughter and her job and I think she did a good job of balancing the two."

When "CSI" went on the air on Oct. 6, 2000, CBS was lagging in the ratings. But between "CSI” and "Survivor," which premiered May 31, 2000, the network's standing improved for the better. Not to mention, "CSI" created a whole new genre of television, which CBS capitalized on with series such as "NCIS" and "Criminal Minds."

"I learned what an impact a television show can have on society," Helgenberger said. "There's the 'CSI' effect on criminal trial juries, but also the amount of young people who were inspired to become criminalists, I never would have imagined that."

"CSI: Crime Scene Investigation" concludes with a special, two-hour series finale Sunday, Sept. 27 on CBS.