'NCIS' Cast Credits Mark Harmon for Having Led Franchise With 'Integrity' Amid 450 Episodes (Exclusive)

ET was on the set with the stars of the long-running procedural as they toast to their 450th episode milestone.

NCIS reaches another milestone: 450 episodes! The long-running CBS procedural celebrates the rare achievement with Monday's episode, and only ET was on the set with the cast to toast to the impressive feat. Though longtime star Mark Harmon exited the series early in the 19th season, NCIS star Wilmer Valderrama credited his former co-star for leading the show to where it is today.

"What NCIS has done beautifully, and I got to say, we're here on the shoulders of so many incredible actors that were here before us, actors who, some are here or some have passed, guest stars who have come and gone, and this show has seen so many careers blossom," Valderrama, who joined the series as Nick Torres in season 14, told ET's Matt Cohen. "So you think about what this show has meant, not only for the fans, but to our industry and what it's done for us as a community of actors who are looking for that break or to be discovered."

"It is also a big tribute to Mark Harmon, who led this beautifully for hundreds of episodes and was able to keep this boat with the integrity and the value set that it still performs today," he credited. "Then come full circle, all of us coming in and being disruptive enough to continue to reinvent it for the audience. So there's a recipe of things that have to really go right in order for you to get to 450. And we can't take credit for the first 300, but I will say that it's beautiful to stand in the same ground as this team."

Added Katrina Law, who joined as Jessica Knight at the end of season 18, "NCIS, the entire franchise, has managed to capture magic in a bottle. And the fact that the show at 450 episodes is still the No. 1 show on television is just a testimony to the juggernaut that is NCIS. To be a part of it and to stand next to this guy [Wilmer Valderrama] onstage and onscreen is pretty great."

In the 450th episode, the NCIS team investigates the death of a ride-share driver who was found after a car accident. It also peels back the curtain of Alden Parker's relationship with his dad, retired Navy vet Roman (guest star Francis Xavier McCarthy), who is temporarily living with him, as he assists the team in their investigation. 

"We're going to have a little bit of a family affair. We might get to see a little bit of Parker's past coming up in the form of his dad," Diona Reasonover, who was introduced as Kasie Hines in season 15, said of the monumental episode, "which is really exciting because we really want to get a chance to explore some of those familial relationships. And we're going to get to see maybe a little headbutting."

"The beautiful thing about 450 is that you have now been with the new team for about a year and some [change], so you're really starting to see us gel and starting to be able to crack jokes at one another. And the characters are getting to know each other," Law noted. "As actors personally, we're getting to know each other. So the freedom to have fun on set is becoming boundless at this point. I think Torres and Knight, we've officially become partners, so we get to share that mantle together. And personally for Knight, you get to see her with Jimmy Palmer and have them hash out some workplace romance."

Sonja Flemming/CBS

As the cast reflects on the series being in rarefied air as they cross the 450-episode threshold, some shared their theories as to why the show -- which is on the heels of Gunsmoke (635 episodes), Lassie (591 episodes), Law & Order: SVU (531 episodes) and Law & Order (481 episodes) as the longest-running scripted programs -- continues to strike lightning in a bottle, two decades in.

"I think that throughout the different iterations of this show, the thing that's remained constant is the characters," Brian Dietzen, who has played Jimmy Palmer since season 1, told ET. "We've had a lot of amazing characters from season 1 until season 20, and they've kind of served to form a family, and I think that their interactions are what keep people coming back. There are a lot of shows that are ripped from the headlines and ours are definitely ripped from the characters. How do the characters react to these crimes every week?"

"Yes, 100 percent," Reasonover agreed. "I think the audience falls in love the same way I fell in love when I came on this show. You just get to watch all these people that you really care about week after week."

Sean Murray, who has been with the show since Day 1 as Timothy McGee, cautioned trying to dissect why viewers remain connected to the stories and the characters: "You don't talk about the no-hitter!"

"It's incredible," he said later. "It's a miracle for a show to get going, to get picked up, to go a year, to go beyond that. To be a part of something like this is just, it's incredible. I've been doing this now 20 years. They can't get rid of me."

Added Gary Cole, who joined as Alden Parker in season 19, "I'm saying it's working. We don't know. But we're still standing here."

And if the cast has anything to say about it, they're gunning for Gunsmoke's ultimate record. (CBS has already picked up NCIS for a 21st season.) "We're coming for you, Gunsmoke!" Cole quipped.

"I think we got it in us for a while. I think we could hit it," said Law, optimistic that NCIS could very well surpass the classic Western that ran on CBS from 1955 to 1975. "I think there's always room for NCIS out there. I think the fans would follow and stick along for the ride."

Valderrama observed that NCIS is one of the few scripted dramas actively airing that could realistically have a shot -- and that, according to him, speaks a lot to the staying power of the franchise. "There's no other franchise in television right now that has a window of opportunity to iconically break any record. And NCIS has been proven that if by 450 episodes, they're still the No. 1 show on television, even if we become the No. 3, you're still going to be picked up for another 10 years. What is the next generation, where does it go from here and where does the baton get passed? But it's a very exciting thought to think that this show can actually go that far."

"You don't know what the future holds," Cole said. "It's really unlikely that a fictional scripted series will ever be in this situation again."

NCIS airs Mondays at 8 p.m. ET/PT on CBS.

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