While no one was looking, a six-year-old television series that lands on few magazine covers, enjoys almost no love from critics and never gets a sniff of a nomination for prestigious awards is suddenly the hottest show on the air.

Yes, that title really does belong to the CBS drama "NCIS": The numbers are undeniable. Last Tuesday that crime show about the Naval Criminal Investigative Service reached 18.8 million viewers, more than it ever had before and more than any other show on television for the week.

But that statistic told only part of the "NCIS" success story. Two weeks ago, CBS decided to fill a weak hour, Friday at 8 p.m., with repeats of "NCIS." In both weeks since, the repeat has been the most-watched show on Friday, with more than 11 million viewers each time.

And the cable network USA, which added "NCIS" repeats to its schedule in September, has seen its ratings soar anywhere it places "NCIS." The show has dominated in its regular spot at 7 p.m., and on most nights it is the No. 1 or 2 entertainment show on cable, drawing audiences of more than 4 million.

Audiences that big get the attention of advertisers as well. Steve Sternberg, executive vice president for audience analysis at the media-buying agency Magna Global, said in an e-mail message, "I think 'NCIS' is one of those rare shows that effectively combines drama and humor, and it hasn't lost a step creatively." He added, "There is a small central cast and closed-ended story lines. That makes it easier for new viewers to tune in."

Competitors privately profess some astonishment at the new popularity of the series because the improvement flies in the face of the recent formula for prime-time success: a grab-people-by-the-throat concept, lots of sex and must-see story elements that compel viewers to watch every episode or lose the thread.

But as shows like that, which tend to be serial dramas, experience a ratings slide, "NCIS" is part of a swing toward more traditional storytelling, the kind of shows that also prove to be valuable assets to their owners because they can play well over and over.

"I don't know if there's a complete swing back to closed-end storytelling," said David Stapf, president of Paramount Network Television, the studio that produces "NCIS." "But those shows certainly seem to be working right now." He added, "I think what really works is the reliability if it."