Just when you think "CSI: Crime Scene Investigation" will go on forever, it ends.

CNN reported that after 15 years, the detective series is finally saying goodbye, with a two-hour finale on Sunday, September 27. The finale, titled "Immortality" will see recent cast members Ted Danson, Elisabeth Shue, and Elisabeth Harnois seeing the series to its end, with special guest stars William Petersen and Marg Helenberger from the original cast.

Helenberger told Entertainment Tonight, "When it came time to do this episode -- which I didn't quite anticipate happening because CSI was going to go on forever, right? -- but I was happy that they brought me back."

The series started in 2000, and despite being almost an "afterthought", by the end of its first season, the Las Vegas crime lab was the No 10 show on-air, and went on to being No. 1 two years later. By the late 2000s, it was the most popular show in the world, with three spin-offs to its name: "CSI: Miami", "CSI: NY", and "CSI: Cyber".

However, with its dwindling audience and schedule shift (from Wednesdays to Sundays), as well as the shortened Season 15 order, it was becoming clear that CSI is on the fast track to its end.

Despite its popularity and influence on television, creator Anthony E. Zuiker attributed its success to a basic human instinct called rubbernecking. He told CNN in 2003, "I think the show is kind of like a car wreck."

He explained, "You want to keep watching it, but you don't want to turn away. And because people know it's not real and it's make-believe, and people are out there solving crimes and giving a great mystery week to week, I think that's what lures viewers to watch."

However, with Season 15 all wrapped up in a bow, what can fans expect from the ultimate CSI finale?

Cinema Blend noted that the show will presumably pick up after the last season's cliffhanger, but there is no clue yet as to how Petersen and Helenberger will re-enter the case. Heleberger warned, "There were some emotional moments in particular. I saw my teammates just gathering up all the evidence of body parts and you-name-it and I just was moved by that."

Series regular Ted Danson told Chicago Tribune that the show will end "by offering the fans who've been loyal so long with an opportunity to say goodbye to the people they fell in love with at the start."

He promised, "It will be very satisfying."