Brows are furrowed on the headquarters set of "Hawaii Five-0." There’s been another murder.

Sure, people are killed all the time on this show. They get shot, blown up and even cooked in an imu for good measure.

But this one isn’t the usual crime-of-the-week. This is a victim they know, obliterated by a blast of 700 small steel balls moving at 4,000 feet per second.

The cast members bear down on their lines, searching for the right shade of serious.

There’s a lot at stake and they know it. This is a final exam for the freshman drama, the last episode of the season. Simply catching the killer won’t be enough if the story arc bombs.

"I think it is always really important to bring it home, but this one is a clincher and everyone is waiting to see what’s going to happen," said the man who plays "Five-0" leader Steve McGarrett — Australian actor Alex O’Loughlin — during a break while shooting the finale last month.

When that episode airs at 9 p.m. tomorrow on KGMB, CBS promises that viewers will be left breathless by a barrage of plot twists:

» The "Five-0" unit might be eliminated.

» Danno and ex-wife Rachel might reunite and move back to New Jersey.

» Kono could be punished for doing the wrong thing for the right reason.

» McGarrett and his nemesis Wo Fat square off again, but who’s the worse for their encounter?

» Disgraced Honolulu police detective Chin Ho Kelly could get his job back, which would mean he’d have to leave "Five-0."

» Even the choice of victim in the vicious blast will be cause for pause.

"There are a couple of events that could fracture the team, and they are pretty significant," said Daniel Dae Kim, who plays Kelly. "There are a lot of surprises in this episode. There are surprises upon surprises."

SURVIVING this first season has been no small feat for cast and crew. "Five-0" shot 24 episodes in its return to prime time — two more than a normal season. The show started filming in July, creating episodes on an eight-day cycle that kept everyone working 12- to 15-hour days until mid-April.

Billed as a reboot of the original series, which ran from 1968 to 1980, "Five-0" became the top-rated new show of the season, gained an international audience and established its own identity, primarily by creating back stories for its characters.

Those stories offered the show’s executive producer, Peter Lenkov, an opportunity to craft a mystery-laden subplot within the police procedural.

"Usually when you write the pilot you want to tell an entertaining story," he said outside the headquarters set on Mililani Street. "You want to know if there is life in the show that your characters can sustain week to week and that there are interesting stories to tell. But for me there was this thread, this mystery that existed that I knew I wanted to have some conclusion to."

The first season’s greatest triumph is that "Five-0" is still on the air, Lenkov said.

"I think it is just remarkable with the competition the way it is and the market getting smaller and smaller," he said. "The fact that you can survive, 24 episodes later, is pretty significant."

Cast members credit the show’s success to the opportunity they’ve had to explore their characters.

"Our show will always be tied to a crime, but when it becomes personal, I think it raises the stakes a little and allows the audience to have a more inclusive viewing experience," Kim said. "I think ultimately, the audience really wants to care about the characters."

The season’s final episodes have underscored that idea, said Grace Park, who plays rookie cop Kono Kalakaua.

"They have been less procedural," she said. "We still have the crime, but we are getting into the heart of ‘Five-0.’ We’re getting into what really matters, which is their team, their relationships, why they are doing this and their own drama."

But fans draw the line at the mundane, Park said.

"Acting is life with all the boring stuff cut out," she said. "They wouldn’t care about Kono buying a new bikini."

O’LOUGHLIN found a wealth of emotions to explore in his character’s back story: the murder of his parents. As the season progressed, the actor reveled in what he could explore. In McGarrett he sees an antihero with secrets.

"I want to play a conflicted character, and Steve McGarrett is becoming more and more conflicted and more and more complex as we go on," O’Loughlin said. "He is very, very driven, and the more I play this guy, the more I realize how driven he is."

From the start, O’Loughlin felt "Five-0" would be a hit, something that eluded his last two shows for CBS — "Moonlight" and "Three Rivers" — which both failed.

"I want it to be a good show because I want to be associated with something good," he said. "I want it to be a good show because I want people to like what I’m doing."

The biggest discovery for Scott Caan, who earned a Golden Globe nomination for his turn as detective Danny "Danno" Williams, was that making a TV show can be a grind. As the season progressed, Caan found himself challenged to maintain what he called "creative sanity."

"If this thing goes on for a while, I want to figure out how to stay excited and how to stay engaged," he said. "This is not by any means a complaint. It’s a way of trying to figure out how to stay energized and — not to sound pretentious — how to be fulfilled and like what I am doing."

With production routines well established and character foibles now familiar to both writers and actors, a second season could be considered an easier stretch. But Caan rejected that.

"You can make it easy, but I think the pitfall of making it easy is you start phoning it in," he said. "If it’s not difficult, it is not going to be any good."

ONE MYSTERY that doesn’t require much sleuthing is whether "Five-0" will return in the fall.

CBS has refused to comment on the subject, noting that it will officially announce its fall lineup on Wednesday at the network’s annual presentation to potential advertisers.

But Lenkov said his staff of writers is already hard at work on season two.

"They are already pretty jazzed up," he said. "We’ve been talking about stories for next year and talking about arcs and where we want to go."

One place they want to go is the neighbor islands.

"After all, this is ‘Hawaii Five-0,’ not ‘Oahu Five-0,’" Lenkov said. "We just went on a scout trip to Lanai. That is a place I am really looking forward to shooting. They have some amazing locations on that island."

The show’s hiatus will be short. The "Five-0" crew will return in early June, and the cameras will likely roll in July, he said.

"You really only have about five weeks to get that first script done, although we know what the first story is," Lenkov said. "It’s not a lot of time, but you make it work. That’s how you make TV shows. It’s a machine, and it has to be kept fed."