Jack Bauer has been through hell and it’s starting to show.

Over the last four years – one day at a time – the superhero of Fox’s “24” has saved humanity from terrorists, beaten confessions out of close friends, battled heroin addiction, discovered his murdered wife’s body and, last year, even faked his own death. All with little regard for himself.

But now, as the hit series starts up its fifth season with a two-night, four-part marathon (Sunday and Monday, 7 p.m.), the stress on Bauer, an agent for the fictional federal Counter-Terrorist Unit (CTU), is finally taking its toll.

“This time, something happens to Jack on a very personal level that pisses him off,” says Bauer’s alter-ego, star Kiefer Sutherland. “Because he’s presumed dead, a lot of the boundaries he was restricted by – by virtue of who he was working for – simply don’t exist now. And he’s mad.”

Until now, Sutherland has only subtly allowed the stress of selflessly saving the world to show on Bauer’s boyish face. This season, however, besides being cranky, our clean-cut action hero is looking downright disheveled.

“The weight on people who are responsible for making decisions that affect so many lives, you can only imagine what that must be like – putting 10 lives at risk to save 1000,” the actor says.

Bauer has been in a constant fight against not only terrorists but often the bureaucracy of his own government.

v”Sometimes bureaucracies are incompetent, just [because] they’re bureaucracies,” says co-executive producer Howard Gordon. “And sometimes Jack has to do things outside the law.”

Gordon notes that while Bauer has a strong moral compass, “he’s sort of politically agnostic. He has a humility and respect for the government on one hand, but a contempt for it on the other … Jack’s strength is in his ability to navigate these really narrow straits, keeping the greater good in mind, even while doing the most loathsome things.”

Having faked Bauer’s death at the end of last season, the new story line was a challenge for the writers. “We had kind of painted ourselves into a corner,” grins Gordon.

Creating a “Jack-centric thriller this year puts him on a collision course with the people who thought he was dead,” he explains.

That world includes his one-time mentor, Christopher Henderson (Peter Weller), who is introduced this season.

“Henderson schooled him on the finer points of counterterrorism,” says Weller. “The backstory is … Jack had investigated some CIA agents and CTU guys. While the other people were brought up on criminal charges, they could never prove anything with me. He’s my protege – who turned me in when I was innocent.”

Weller probably has a better understanding of Bauer’s character than most. His father flew President Johnson’s helicopter in Texas.”

“This show really gets it,” the actor continues, “about how the bureaucracy of the United States gets bogged down in minutia, and yet it can all be severed by one phone call from the C.O.”