HOLLYWOOD - Jack Bauer, America's favorite counterterrorism agent with the violent code of honor and the weird sadomasochistic bent, is squaring off against a stealthy and unforgiving new enemy.

His fans.

After peaking in the ratings last year, Fox's thriller "24" has been getting dumped on in its sixth season. Critics and fans are aiming tomatoes at the stage, carping about the soapy and repetitive plotlines that unspool Jack's unlikely familial past, tiresome romantic triangles in the security bureaucracy, and endless bickering among Oval Office advisers.

Last week, with a fresh episode designed to lay the groundwork for what the creators promise will be a typically suspenseful finale, "24's" ratings in the key young-adult category swooned to their lowest level in more than three years, with a total audience of just 10.4 million, according to Nielsen Media Research.

More than one-third of viewers have bailed since the four-hour season premiere in January. And "24" was one of the shows that the Federal Communications Commission singled out recently in urging Congress to curb TV violence.

The protests have not escaped the attention of the show's producers, who promise that some big changes are on the way for Jack (Kiefer Sutherland) and other regulars next season.

"It hurts to hear the criticism," said executive producer and writer Howard, as the cast and crew worked to finish shooting the season's final episode, set to air May 21. "I don't dispute it's been a challenging season to write for us. But it's reinvigorated our determination to reinvent the show. This year could be seen to be the last iteration of it in its current state."

Reinvention? That sounds ominous. But Gordon says not to worry, as Jack, he says, "won't be flipping burgers."

"It won't be a musical or a half-hour," he added. "I've got a couple ideas, none of which I could even begin to share responsibly."

Premiering less than two months after the terrorist attacks of Sept. 11, "24" initially amounted to barely a blip on the pop-culture radar. The premise - each episode unfolding in real time over the course of a single day as Jack races to foil some dastardly conspiracy - sounded gimmicky.

But Fox stuck by the show, and it steadily grew a fan base that made it blossom into hit-level status during the critically acclaimed and Emmy-winning fifth season.

I always loved "24's" willingness to go to crazy extremes in expanding the thriller format and somehow live to tell the tale.

But two personal anecdotes brought the show's mass appeal home for me: My 70-something mother-in-law, a Republican with narrow TV tastes outside of "The O'Reilly Factor," confessed that she never missed "24." And last year I observed a middle-age man take his female companion's hand and inquire, in a voice soothing and conspiratorial, "What do you say we go home, build a fire and watch '24'?"

But the clock is ticking, for fans as well as for Jack Bauer. Longtime devotees are struggling to keep the faith during this trying season.

"The writers have recycled some plots this season that are glaringly obvious: a recording, an almost removed president, an assassination attempt on that president, an attack on a Middle Eastern country, an impending nuclear strike, a person close to Jack kidnapped, etc.," Victor Lana, a novelist who follows "24" for BlogCritics Magazine, wrote in an e-mail. But, "The bottom line is that we still care about Jack Bauer."

Meanwhile, "24's" audience is getting noticeably grayer, typically a sign that a show is losing its purchase on the windy crags of pop culture. According to Brad Adgate, senior vice president at the New York advertising company Horizon Media, the median age is 47.4 so far this season, compared with 45.1 last year and 42 in the 2003-04 season.

But Gordon said he and his writing staff were wondering if something else was afoot besides the normal cycles of story telling and network scheduling.

Could it be that the vague but gnawing post-Sept. 11 fears that helped turn "24" into a hit are ebbing?

"It's something we talked about at the beginning of the season," Gordon said. "Nine-11 is becoming, quietly, a memory; the memory is starting to fade. … I do think that people are looking at the world differently, with less fear."