What's the secret of its success? Certainty. In a world of danger and chaos, CSI told us there will be an answer – that any problem can be solved by scientists who arrive, shine a blue light on the scene, and dispense a solution within an hour.

CSI started at a point in history when we desperately needed that reassurance – just after the World Trade Centre attack in New York and just before the bombings in Bali. It became Australia's most watched series in 2002, 2003 and 2004, and stayed in the annual top 20 for another five years, along with CSI: Miami, until its chief scientist, Gil Grissom (William Petersen), went off to study insects in the jungle, followed a year later by his deputy, Catherine Willows (Marg Helgenberger), who went to join the FBI.

Tomorrow night on Channel Nine, Grissom and Willows are back as central characters in the two-hour finale, titled Immortality and described by Ted Danson (who plays D.B. Russell, the series' most recent replacement) as "one for the fans".

It will be satisfying "for all the people around the world to be able to not just have this kind of, `Oh, it's over', but to have a two-hour ending, where you get to have all the people you loved, Billy and Marg, coming back," Danson says. "They started this show, so for them to end it, that's kind of quite wonderful. There's an amazing explosion, there's a crime scene extraordinaire, there's all of that, but it's also a goodbye to these characters. It's a chance to wrap up relationships and to fill people in on where they've been."

Petersen reflected on what made Grissom such an attractive character for viewers: "The most important thing about Grissom, and in many ways about CSI, is that he believes that what is is, and that that's the important thing. The evidence tells the truth, and when we started the show, I think there was a lot of sort of post-modern vagueness to our world. And what Grissom and the CSIs were able to do is define what's real – that this is what this is, and this is what that means. It's a fairly forensic argument that I think is important in our lives. I think we need that."

Petersen hopes CSI helped to convince people around the world of the value of scientific analysis. The finale reveals that Grissom "put the tents away", he says. "Maybe he got too many mosquito bites or something while he was in the jungle, but he headed for the ocean. I think what happened is that he realised how desperate our oceans have become and the mammals in them, and I think he felt called to see if he could do something about that. He's an activist for the oceans."

Helgenberger says she wanted to give "closure" to Willows because "she's like the grounding force of my career".

"Catherine Willows is based on a real person, by the name of Yolanda McClary. She was with the Las Vegas Crime Lab for years and years. I connected with her – Yolanda – and I just really connected with Catherine, and loved playing her, and I'm glad I'm getting this one last shot of putting on her suit and her boots and, you know, that swagger."

Helgenberger was first attracted to the role because Willows was "a single mother, ex-stripper, and I thought, `Well, there's a lot to play there" – the way she handled herself and the way she had some femininity and she had spunk, and she didn't have to, I guess, deny any kind of sexuality just to be with the guys".

Danson is delighted that his character will be shifting to CSI: Cyber. "He's turning over a new leaf. He's getting divorced, and that's huge. CSI: Cyber is a bit lighter because you're not in an autopsy room. You're doing autopsies on machines and computers and cyberspace. You may not be dealing with dead bodies, with a human body, but you are dealing with computers, which have a brain …

"So you still have a body, but the body now is a computer, so my job is to gather all these new gizmos as they come on the market and test for vulnerabilities to stay ahead of the bad guys, but I'll also be a thinker that's part of the team trying to figure stuff out."

CSI: Immortality airs on Monday, November 23, at 9pm on Channel Nine.

The creation of CSI

Anthony Zuiker, the son of a waiter and a blackjack dealer, was driving a tram around Las Vegas, on a salary of $US8 an hour, when he thought up the idea of a TV series about scientists who help police work out what has happened at crime scenes. His pitch was rejected by three United States networks before getting a trial run with CBS, which gave him three days to write a pilot script.

"CSI being my first TV script, the last thing on my mind was, `Is this a format that can last for 16 years?' " Zuiker told Variety magazine last month. "I was new to television and new to the industry, but me being from Las Vegas and being a risk-taker, the one thing I wanted to do was come in to TV and break every rule I possibly could.

"We did flashbacks and recreations … We constructed this format that crystallised the procedure. You could afford to do anything as long as you had great writing and great mysteries."

CSI went to air in the US in late 2000 (and early 2001 in Australia). It was so successful that in 2002, CBS commissioned a spin-off called CSI: Miami, which made a legend of sunglass-wearing cop Horatio Caine (David Caruso). That lasted until 2012. Less successful was another spin-off, CSI: NY, starring Gary Sinise, which ran from 2004 to 2013. This year CBS launched a further spin-off, CSI: Cyber, which stars Patricia Arquette as real-life cyberpsychologist Mary Aiken. Ten shows it in Australia.

The original CSI won three Emmys as Outstanding Drama Series, two Emmys for Marg Helgenberger as Outstanding Actress, three Golden Globes as Best TV drama, and acting Globes for both Helgenberger and William Petersen.

For more, see The Tribal Mind.