Numerous unexpected factors have played a role in this surprising turnaround, some of which are not likely to last, but the numbers are clear. Among the viewers prized by most advertisers — 18- to 49-year-olds — NBC has beaten its network rivals every week of the new television season.
That’s three weeks, and NBC is in contention to post a fourth victory when the Nielsen accounting is official on Tuesday. (CBS, as it has for years, remains well ahead in terms of the total number of viewers.)
NBC’s ascent is the most striking development of the season, but it goes hand in hand with a wider story of network performance that, NBC aside, has ranged from disappointing to alarming.
While NBC has managed to increase its 18-49 number an impressive 15 percent, ABC is down 12 percent, Fox is down 19 percent and CBS — which had a huge start last fall because of curiosity surrounding the departure of Charlie Sheen from “Two and a Half Men” — is down 24 percent from the three-week period a year earlier.
The two developments are linked: NBC has been able to ascend at least in part because its competitors have descended so sharply.
Even at NBC, few people expected this performance. “We didn’t dare dream we’d win the first three weeks of the season,” said Stephen B. Burke, the NBC chief executive. “We were pretty sure we would do better, and had laid the groundwork and had the strategy to do better.”
Robert Greenblatt, in his second season heading NBC’s entertainment division, said, “I was hoping we’d be out of fourth place and comfortably in third. But we’re comfortably in first.”
NBC came into the season with a limited strategic objective. “Our big goal was to build one night a year,” Mr. Burke said.
With National Football League games making Sunday night an all-but-guaranteed win, the idea was, “O.K., we’d like to try to win Monday night,” Mr. Burke said. “And we’ll try to be competitive on Tuesday.”
Thus far, NBC has won every Monday and Tuesday to go along with its football-fueled Sunday.
The crucial decision was adding a second edition of the hit singing competition, “The Voice,” on Monday to complement the version that ran in the spring. “There were people saying we brought it back too soon,” Mr. Greenblatt said.
The outcome of that move has yet to be determined.
“This is the first time a singing competition is going to have two arcs in the same season,” said Brad Adgate, senior vice president of research for the media-buying agency Horizon Media. Fox’s “American Idol” has always stuck to one edition a season.
But “The Voice” has continued to be a potent draw, easily eclipsing Fox’s fall-season singing show, “The X Factor.”
The other essential ingredient in NBC’s early success has been the addition of the season’s only breakout hit, “Revolution.” That drama, about a postapocalyptic world without electrical power, has not only dominated its 10 p.m. Monday time period, it has added more than 50 percent in the 18-49 audience when delayed viewing is included.
Mr. Greenblatt said that having two hours of “The Voice” on Monday would not have been enough to turn the night around if NBC had not developed a show that could take advantage of that big lead-in audience. “You have to have a show that the audience truly wants to watch,” he said.
At this point last season, NBC’s Monday consisted of two hours of “The Sing-Off” and a new drama, “The Playboy Club.” That lineup averaged 4.4 million viewers and a 1.6 rating in the 18-49 audience. This year’s lineup has averaged 11.3 million viewers and a 4.2 rating.
“The Voice” has also set up improvement on Tuesday night by attracting winning ratings from 8 to 9, and driving viewers to two new comedies, “Go On,” and “The New Normal,” from 9 to 10.
Frontloading the week was a centerpiece of the strategy. NBC used the big audiences it attracted this summer for the Olympics to promote its new shows, even sliding a preview of “Go On” into its coverage one night. The preview pulled in 16 million viewers.
“The Olympics helped us,” Mr. Greenblatt said, noting that NBC rushed many of its new shows on two weeks before the other networks. “It was smart not to let the platform of the Olympics die out completely.”
But the other networks may have helped NBC’s cause by offering little new to excite younger viewers. “It has not been a stellar season for first-year shows,” said Mr. Adgate of Horizon Media.
Last fall, new shows like “Two Broke Girls” on CBS, “New Girl” on Fox and “Once Upon a Time” on ABC were all instant hits, but nothing at that level has emerged on any of those networks.
Fox replaced the longtime favorite medical show “House” with a much-derided offering called “Mob Doctor.” CBS replaced “Two and a Half Men” on Mondays with a comedy called “Partners” that has found little traction. ABC’s longtime Monday powerhouse, “Dancing With the Stars,” has been overtaken by “The Voice” and has lost a significant portion of its younger viewership.
The greatest threat in the battle for younger viewers is the continuing appeal of top cable dramas. AMC’s “The Walking Dead” smashed everything else on television in drawing young viewers for its premiere last week. Other cable series this fall, like “Sons of Anarchy” and “American Horror Story” on FX, are attracting more coveted young viewers than many network shows.
Mr. Greenblatt said every network had noticed the numbers posted by “The Walking Dead.”
“I’m scrambling around to see if we have anything high-concept like that in development,” he said.
The show most like that is “Revolution,” a series Mr. Greenblatt described as “a big new idea — which is where I think you need to be in drama.”
But the networks have offered little else this fall containing the new-idea elements of twisty plotlines and serialized storytelling, two of the hallmarks of cable drama. (Fox has a contender in that category coming in January, an intense serial-killer drama, “The Following.”)
“It doesn’t make sense for the networks to put on another police procedural if they are trying to get younger,” Mr. Adgate said. “They need shows that get people talking on social media.”
Mr. Adgate said NBC deserved credit for its fall strategy, though he added that the gains might be hard to sustain. “In the first quarter, they won’t have football,” he said.
CBS is likely to get back on track after the new year, partly because it always does because of a largely stable schedule. But the main reason is that it owns rights to both the A.F.C. championship game in prime time and the Super Bowl.
“We’re certainly not declaring victory, because we know it’s long term and the season goes in cycles,” Mr. Greenblatt said. “But to have this kind of strength at the beginning of the season is really unexpected.”