NEW YORK – For Jack Bauer, saving America is all in a day’s work.

But early in this new day on the Fox thriller “24,” his calling as the counter-terrorist go-to guy has clearly taken its toll.

“Tell the president I’m sorry,” Bauer sobs into his cell phone just four hours into the current daylong ordeal. “I can’t do this anymore.”

By now, even staunch “24” fans may feel the same way, having suffered with Jack – and with the preyed-upon nation whose very survival depended on him – for the past five seasons.

But he will carry on, of course. And so will we, answering the call of the explosive first four episodes of “24” when they air Sunday and Monday from 8 to 10 p.m.

Two years have elapsed since last season’s crush of crises du jour, a day that saw Bauer (Emmy winner Kiefer Sutherland) bring down treasonous President Charles Logan – and then, in a cruel twist, get kidnapped and thrown in the hold of a China-bound tanker to face punishment for raiding a Chinese consulate.

The present day finds the United States in turmoil as, moments after 6 a.m., an L.A. city bus is blown up by a suicide bomber. Thought to be the work of Islamic militants, it’s the latest in a series of bombings that have pushed Americans to the brink of hysteria.

“They’re afraid to leave their homes,” says President Wayne Palmer (younger brother of former President David Palmer), reaching out to Bauer in desperation. “They’re actually starting to turn against each other.”

The president has managed to get Jack sprung from the Chinese and returned to L.A. He needs him for a quid pro quo to stop the carnage.

It will mean (what else is new?) Bauer’s almost certain death.

“It will be a relief,” says traumatized, tormented Jack.

But relief is something Bauer never gets. Always in motion and obliged to cheat death, he’s a slave to against-all-odds endurance. For the sake of America. And at the price of high anxiety for “24” devotees.

It’s been that way since the series premiered.

“24” took flight from an ambitious if gimmicky concept: a multi-strand narrative of action and intrigue whose indefatigable hero would fight domestic terrorism in an hour-by-hour, real-time rush tracking a single day that would span a full season.

But then, just weeks before its premiere, “24” received unbidden, awful validation: the events of 9/11. Indeed, the series got off to a particularly uncomfortable start when, on its first episode (aired Nov. 6, 2001), a terrorist blew up the jetliner in which she had been a passenger, cleverly parachuting to safety.

Through no fault of its own, “24” arrived seeming far too close to real life. The hopped-up dread that propelled “24” must have struck many viewers that first season not as sleek escapism, but as quite the opposite: a disturbing ricochet off their own altered world.

The first season, “24” won critical acclaim as Bauer foiled an assassination plot against David Palmer, then a black U.S. Senator on the fast track to the Oval Office. But “24” wasn’t a hit. It ranked 74th place in viewers.

Its ratings ascendancy has come in recent years (last season it was tied for 24th), maybe owing to the passage of time since 9/11.

But, more likely, it’s thanks to the series’ knack for somehow raising the apocalyptic stakes of each crisis Bauer confronts. And to the growing assurance with which “24” somehow straddles the extremes of hyper-real and preposterous: “24” is patently absurd on so many levels, yet it’s as white-knuckles gripping as anything on TV. If we look too close, it’s laughable. But it cuts too near the truth to not keep watching.

“24” fans were blown away by last season’s fearsome frolic with President Logan. A foppish wretch, he turned out to be behind a cockamamie plan to manufacture deadly nerve gas and sell it to foreign terrorists. But then they threw him a curve by making Los Angeles, not Russia, their intended target. Compounding his villainy, Logan had a hand in David Palmer’s murder.

Now it’s a new day dawning, and if 6 a.m. through 10 a.m. proves anything, this season will up the ante even further.

Created in a nation that no longer exists, “24” has deftly adapted to the post-9/11 era in which it’s enfolding. When the ground shifted beneath it, “24” shifted, too: from a series that would dramatize the unthinkable, to an all-too-thinkable vision of some day looming soon.

“24” is a wildly idealized view of our nation’s response to the threat of terrorism on our soil, yet – even within the series’ tidy 24-hour window – it has thus far withheld easy answers and happy endings. Jack Bauer, the nation’s point man for homeland security, is valiant but steadily unraveling.

So “24” triumphs as a series it surely never set out to be: an exceptional adventure about lowered expectations. Its message is clear: Prevailing is too much for a nation to hope for. At the end of the day, endurance will have to do.