The CBS drama "NCIS" (Tuesday night at 8) joined a select group of prime-time TV shows recently when it aired its 200th episode. Only 62 series -- out of the thousands that have filled network and cable channels -- have reached the milestone.

And "NCIS" shows no sign of slowing down. It's consistently the highest-rated drama in the weekly ratings and has 13.7 million Facebook fans. The popularity is global: It airs in more than 200 international markets and is heard in 20 languages.

The show appeals to more than just older viewers. It has higher ratings with viewers ages 18-49 -- a demographic advertisers love -- than "Glee," "Dancing With the Stars" and "The Office." So how does a series about the weekly exploits of a team of Navy investigators become such a hit? Series star Mark Harmon says it started with mediocrity.

UNDER THE RADAR "I think, from the very beginning, we were a show that wasn't good enough to get all that noticed and wasn't bad enough to get canceled," Harmon says.

That might sound strange, but he has a point. TV shows that start as big hits have a tendency to fade quickly, such as "Twin Peaks" and "Glee." Series that start slowly rarely last past a few episodes, such as "The Playboy Club." Because "NCIS" started out in the middle of the viewing numbers, the series had time to build a solid foundation of story lines and characters.

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CHARACTERS COUNT "NCIS" is loaded with characters, from the gruff boss played by Harmon to the quirky lab rat brought to life by Pauley Perrette. Toss in a nerd (Sean Murray), a swaggering hero (Michael Weatherly), a suave British doctor (David McCallum) and a hot foreign agent (Cote de Pablo), and the show has a host of interesting places to go.

Executive producer Gary Glasberg credits series creator Don Bellisario for establishing such a versatile character blueprint.

"There was a foundation that he started with, a brilliant foundation of story and character that was there. And over the years, we just sort of kept scratching the surface of it. And to be nine years into a show now and still have so much to work with is a real comment on what Don came up with to begin with," Glasberg says.