Presidential assassinations, nuclear warheads, biological warfare, international terrorism -- they've all been in a day's work for Counter Terrorism Unit operative Jack Bauer on Fox's "24" for the past four seasons. Of course, in the chronology of the show, they've really only been four very, very long days -- and Sunday's first installment of a two-evening, four-episode fifth-season premiere will kick off another extraordinary day in the life of Agent Bauer, played by Kiefer Sutherland. But no matter what happens to him, it could hardly be more mind-boggling than the fact that "24," a show initially pegged as a one-season trick pony, has turned into a full-blown success story and model for using DVD and international revenue streams to keep a struggling broadcast series afloat. Yes, perhaps the most surprising development in "24's" history might occur on Monday night, as the series hits another magic number: 100 episodes.

"There were some who dismissed the format as a gimmick at the start," co-creator/executive producer Joel Surnow recalls. "We had a gimmick like (NBC's) 'Law & Order' had a gimmick. It was just a change in the way you could produce a drama for television. Because we were still telling basic crime stories like any other show, it wasn't really a stunt. But what we did, I think, is modernize the storytelling in a way the audience had never before experienced it. (Since) we also got to recast a new story each season, it was like people were watching a new show every year. I think it explains why we're still here."

That "24" has lived to see 100 and beyond is a testament to the fact it has quite literally stood the test of time. The show has always been a creative challenge: It is the first TV series to embrace a real-time format (each episode representing a single hour in the same day, running chronologically through a 24-episode season, no jumps forward or back in time, with each episode named for the hour in which it takes place), and it weaves its dramatic yarn by the use of split screens to tell multiple stories simultaneously, a visual conceit rescued from the hip basement of 1960s films like "The Thomas Crown Affair."

"Our first director, Stephen Hopkins, established the tone very early by conveying chaos but without a lot of extraneous bells and whistles cluttering the frame," says Rodney Charters, who joined "24" as its director of photography following the pilot episode.

"The mandate was, first off, to make the lighting and frame of reference totally believable," he continues. "There would be nothing blinking or dramatic jazzing things up and drawing attention. To capture that dynamic, we've always covered a scene from beginning to end in a single take. Strictly from a physical standpoint for the camera operators, it's endlessly challenging. But watching the dailies, we were able to see how that visual style draws the audience into this world. Then Kiefer's intensity drives it out of the park."

Today, such creative leaps in storytelling and presentation are lauded by network executives. Notes Fox Entertainment president Peter Liguori: "Much in the same vein as Tom Clancy's ability to crank out tense novels, (executive producers including Robert Cochran, Joel Surnow and Howard Gordon) have this unique genius of being able to create really taut situations. The audience taps into that level of tension in a big way, and it continues to accelerate and get better."

It was not always so. The series has managed to defy long odds and break virtually every rule in terms of creative execution, audience expectation and means of production -- despite early concerns up at the top. The serialized storytelling that has become the show's hallmark was initially thought to be the least-savvy business practice imaginable. Conventional wisdom held that only stand-alone episodes could sell in syndication, and few shows could work less well than "24" as stand alones.

While the show's unconventional approach has worked in the long run for both Fox Broadcasting and producer 20th Century Fox TV -- enough to earn it a two-season renewal in May -- there was concern following the end of the show's first season in 2001-02 that it would even be renewed, based on household ratings averaging an 8 share.

"As we looked at a second season, the question was posed, 'Can we dare do this again?'" Gordon recalls. "It was, and is, such a high-wire, fly-by-the-seat-of-your-pants tightrope act. We thought, 'Maybe let's just finish out a season and be a noble failure.'"

Dana Walden, co-president of 20th TV, recalls that the conversation "wasn't so much how the show could come back but whether it could possibly sustain another season with this concept."

Cochran and Surnow considered revamping the format for Season 2, deciding to cover 24 hours each week rather than over the course of a full season, but Walden says, "It was good, but it wasn't '24.' The network deserves a lot of credit for taking the leap of faith and bringing it back for Year 2."

Perhaps what saved "24" after that first season -- in addition to Fox's faith in the show -- was a strong international appetite and, later on, A&E's decision to reair the show in marathon blocks. Then came the prescient decision to release Season 1 on DVD prior to the premiere of Season 2, an almost unheard-of concept, as both marketing tool and revenue stream for a broadcast network. Sales of that first "24" DVD set considerably exceeded projections (see sidebar on page S-10), and the show became profitable, despite some decidedly modest ratings numbers and its inability to effectively repeat.

"That, combined with the fact the network had given us a very low license fee, created a whole new revenue base that offset the high cost of production; (it) makes a series profitable," Walden adds. "A lot of people have been very inventive in the way this show has been marketed. It continues to pay off."

Not that "24" is an inexpensive show to produce. Quite the contrary, it's the rare show that is actually filmed in the city where it is set: Los Angeles. That makes for a pricier proposition than if the show were made in, say, Toronto or Vancouver, though the producers doggedly maintain that to make it in Canada or elsewhere would greatly diminish the show's high-octane drive and electrifying mood.

Indeed, on the production side, the "24" creative team has continued to keep the concept vital and fresh and its devoted core of viewers off-balance. Not only does the series seem to be reinvented within the context of its format each year -- as a mind-numbing array of obstacles are planted in Bauer's path -- but the producers also have taken another leap with its characters. That is, they kill them off -- series regulars alongside of the bit parts; Season 1's real heart-stopper came when Bauer's wife, Teri (Leslie Hope), was offed.

It's a twist that makes the job of the show's casting directors, Debi Manwiller and Peggy Kennedy, particularly complex. "Everything about this show is unpredictable," Manwiller says. "There are actors on hold, the story line is constantly changing and there's no real certainty where things are heading. We wind up having to take guesses. But you know, that's part of the fun."

Perhaps surprisingly, even producers and writers say each season is not mapped out from Episode 1. Executive producer Evan Katz says that each season's story line is driven by a master plan at the outset, but after the first four or six episodes, "We make a lot of it up as we go along."

It's really more like making 24 mini-movies every year, asserts Jon Cassar, who will direct roughly half of the coming season's "24" episodes and who presided over 34 of the first 100. He describes the show's unique shooting style as "a mixture of documentary and theater."

While most TV dramas film scenes as a collection of separate shots, on "24," each scene is composed as a seamless piece that keeps the camera rolling from start to finish. "Hardly anyone notices, but the whole show also is shot at human height," Cassar says. "There are no high or low shots; that lends this show a voyeuristic quality that sucks viewers in."

The real key has been in those viewers, many of whom first found the show on DVD. Audience numbers have increased steadily since that first year, jumping to double-digit share levels over the past three seasons (10, 11 and 10, respectively) and achieving a 7.2 household ratings high for the show in the fourth season.

Additionally, Fox has smartened up about how to schedule the show, launching each new season in January rather than the fall, presenting it with no weeks off or reruns. According to Katz, that change is "a brilliant innovation that has made a huge difference. There are no weekly gaps in the presentation, which takes a demanding and very intense show and makes it much more attractive to the audience."

This season will no doubt require an even greater intensity by the audience and writers, though little information has been made available. The show's brain trust is fairly paranoid about plot points, but here's what can be said: Sean Astin, Jean Smart, Connie Britton, Julian Sands, Peter Weller and JoBeth Williams have joined the show, which picks up 18 months after Season 4.

"I can tell you that this year, we've actually decided to go smaller with the story line as compared to last season," Gordon says. "We'll see a collision between Jack's former self and his current self. And he'll be at the center of the action as opposed to on the outside looking in as he has the past few seasons."

If there is any call-out to be made against "24," then it should likely be in the very way its plot twists and turns simply defy reasonable explanation or plausibility. How could this many horrible things happen to one agent in the course of a single, absurdly God-awful day and night? More importantly, does Bauer never have to use the bathroom?

Surnow feels the pain of those who take issue with the show's believability, but he suggests the show is best enjoyed when disbelief is checked at the door. "The idea that we strain credibility with all of the things that happen in a day is valid," he admits. "But I have to believe that if we do our job compellingly, the audience just doesn't care. So far, I think it's turned out pretty well."