Mark Harmon, who stars as Special Agent Leroy Jethro Gibbs in the CBS series “NCIS.”

O.K., maybe other people are more deserving of sympathy. Still, it can be a struggle to get a quietly successful show noticed by the wider television world, which may explain why CBS has turned episode No. 200 of “NCIS” into a cause for celebration.

Its producers and publicists have been planning for this Tuesday’s milestone episode of this criminal drama since last spring. The studio that makes the show wheeled out a cake and held a ribbon-cutting on the set last month when the episode was being taped. And the network has been promoting the big round number in commercials and on Facebook in recognition that so few one-hour dramas ever make it this far.

“The fact that we hit this number at the same time that we’re No. 1 with viewers is extraordinary, and there isn’t a single person here who takes it for granted,” said Gary Glasberg, the executive producer and show runner who oversees each episode.

The episode on Tuesday is a kind of reward for longtime fans, bringing back some past characters for cameos and setting up some what-if scenarios for Leroy Jethro Gibbs, the Naval Criminal Investigative Service special agent Mark Harmon has played since the show’s inception, in the fall of 2003. “It’s all about the ripple effects of decisions and choices he has made throughout the years,” Mr. Glasberg said.

Nina Tassler, the president for entertainment for CBS, praised the producers for being “very mindful of the fans that have been there since the beginning” by revealing more about the characters bit by bit.

“NCIS” nowadays is “like a supernova,” she said, netting 22.7 million viewers for new episodes this season, up slightly from last season, which was its highest-rated to date. It has been the most-watched scripted television show in the United States since the 2009-10 season, when it surpassed “CSI,” another CBS franchise.

And yet it is also awfully unassuming. The series is shot in the canyons of Santa Clarita, Calif., north of Los Angeles, mostly out of sight and mind of Hollywood. It retains an unusually high number of its staff members each season. It meets its deadlines. “This is not a set where the size of your trailer is important,” said Mr. Harmon, who wasn’t about to name the sets where it is important.

The show had a humble beginning, with about 12 million viewers on average, middling by CBS standards at the time. “We were not good enough to be paid attention to and not bad enough to be canceled,” Mr. Harmon said.

As a spinoff of “JAG” — those letters stand for Judge Advocate General, a legal branch of the Navy — which ran between 1995 and 2005 and totaled 227 episodes, it was allowed to grow slowly, and it did, flouting most of the trends of network television. (Oddly, “JAG” itself did the same — it was canceled by NBC after one season, but revived by CBS and made into a hit show.)

“NCIS” was especially popular early on in other countries. “The international audience was ahead of us,” said David Stapf, the president of CBS Television Studios. In the United States 2008 is perceived to be its breakout year; that’s when the cable channel USA started showing repeats of the series, which rated exceptionally well. The producers believe that the cable reruns furthered the show’s popularity on CBS, too, since it has continued to grow since then.

Back in 2003, when it had its premiere, Mr. Stapf said, studio executives wondered if the writers would have enough material to go the distance, since the real NCIS only investigates crimes that affect the United States Navy and Marine Corps — a smaller scope than, say, a metropolitan police department’s. “But we quickly got over that,” he said.

The executives also worried at first about the humor interwoven into the drama’s plotlines. As Mr. Stapf put it, “Can people be joking when they’re standing over a dead body?” But the humor, he added, “also made the characters very real and very relatable.”

“This is their job,” he continued. “They stand over dead bodies every single day.”

While the dead bodies lent the show a procedural formula, the laughs helped to highlight the characters: both the boss, played by Mr. Harmon, and the agents who work for him. The characters and their relationships have become pivotal parts of the show.

Now there is also a spinoff, “NCIS: Los Angeles,” which is drawing about 18.4 million viewers on average this season, and there are regular marathons on USA.

“It’s not unusual for people to say to me, ‘Hey, all day yesterday, I was watching the marathon,’ ” Mr. Harmon said. “It makes me laugh. I’m like, ‘Get outside, do something!’ ” But he added, “It’s so important to realize the effects that the show has on people.”

For Mr. Glasberg there is a lesson embedded in “NCIS” about slow, steady character building. “You can give people a little bit, and it satiates them and leaves them wanting more,” he said.

This winter one of the lighting technicians came up to him and said: “ ‘Gunsmoke,’ right? We’re going for ‘Gunsmoke?’ ” That series lasted on television for 20 seasons and 635 episodes, albeit at a time in television history when viewers had far fewer choices than today.

Mr. Glasberg said he answered, “I’ll do my best.”

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