Kathryn Bigelow's gripping docu-drama "Zero Dark Thirty" is technically about the 2011 mission that resulted in the killing of Osama bin Laden, the terrorist leader who brought so much sadness and fear to America and the rest of the world. But that's not what interests Bigelow and screenwriter Mark Boal. We know how that story ends.

What interests the two Oscar-winning filmmakers is the journey that brought us to that point. The duo calmly and chillingly chronicles the dark and exhausting trek our nation took into a moral grey zone. It's a path that includes the loss of thousands of innocent lives, two bloody wars, a country in fear and ethical conflict. We may have come out with our mission accomplished, but was it worth it? Is the scab created by so many lives lost and morals bent healed? And where do we go now?

These are the questions that haunt Bigelow's terrific procedural. They haunt the audience well after the end credits have run and the popcorn has been thrown out. They also make "Zero Dark Thirty" one of the most thrilling and thought-provoking films of the year.

Oscar nominee Jessica Chastain stars as Maya, a young CIA operative sent to Pakistan to put her knowledge and research into bin Laden to use in the hunt. When we first meet her, she is witnessing a prisoner getting tied up and waterboarded for information. She's clearly rattled but unwavering. Jason Clarke, chilling without becoming villainous, plays Dan, the man in charge of the process.

After several more bouts of waterboarding, confinement, starvation and shaming, the prisoner refuses to crack. Constantly pressed for time, Maya and Dan eventually try a new tactic: taking the man for lunch and using old-fashioned trickery. The plot works, providing Maya with the name of bin Laden's key messenger, Abu Ahmed, the first crucial clue in the laborious manhunt.

The use of torture and EITs – enhanced interrogation techniques – in "Zero Dark Thirty" has seemingly created more anger and controversy than the potential use of the same tactics in real life. However, the facts are simple. Bigelow's film does not promote torture. Portrayal is not the same as promotion, and, as presented by Bigelow and Boal, the inhumane tactics are uncomfortably intense, haunting, borderline unbearable and not all that useful. In fact, almost every other tactic is more effective for our protagonists – that is, if we can really call them that.

The movie opens with the panicked, horrifying sounds of Sept. 11. The audience's collective wound has been reopened, ready to be healed by heroes. Instead, we get operatives with their sense of humanity blinded by their mission. Bigelow's camera and use of Chastain's character as the audience's link into the story makes the viewer complicit, making it even more unsettling to watch.

Does any of this sound like a glowing endorsement for torture? Even if one argues the tactics used in "Zero Dark Thirty" lead to bin Laden, the final goal is unrewarding. Once again, was it worth it?

Back to the story. The information gained eventually leads Maya onto a massive and intricate trail of leads, locales and dead ends. The project is her sole existence and obsession, even while its death toll, frustrations and urgency make it hard to cling to hope and stay enthusiastic about her one true purpose. Eventually, thanks to some crafty spy work and research (not torture, mind), they find the terrorist leader's compound. The rest – Seal Team 6's successful raid on bin Laden's compound, led in the film by Joel Edgerton ("Warrior") and Chris Pratt (TV's "Parks and Recreation") ­– is now history.

Chastain, who has been growing into one of Hollywood's finest actresses with great roles in "Take Shelter," "The Tree of Life" and "The Help," is the emotional glue that keeps the story in "Zero Dark Thirty" together. Watching Maya subtly evolve throughout the movie, growing more and more into her professional obsession, is captivating, and Chastain plays every scene perfectly. She's wonderfully expressive in smaller, more nuanced scenes but still more than capable during her character's big bursts of emotion.

Maya is the audience's window into the story, and well as one of the film's lone key characters, so a strong lead performance was crucial. With the wrong actress, she could have ended up being a cliche, plucky heroine. Luckily, Chastain delivers one of the year's best. It always helps to be surrounded by such a solid supporting cast as well, including Clarke, Kyle Chandler, Mark Strong and James Gandolfini.

It's an easy connection to make between Maya and Kathryn Bigelow. They're both strong women, excelling in fields normally considered boys' clubs. It's an interesting connection, but that's not why Bigelow succeeds with "Zero Dark Thirty." She's proving yet again that she's not just a great female director. She's just a great director, period.

Bigelow and Boal previously hit Oscar gold with "The Hurt Locker," and if anything, "Zero Dark Thirty" is a step up. Both create a film of fascinating authenticity. There's never any doubt, even when recognizable names show up, that what the audience is witnessing is what really happened. The investigation, the tactics, the dialogue, the hard-edged characters and even just the offices all feel pulled straight from reality, especially the climactic raid, which shows one need not sacrifice realism and pacing for the sake of intensity and thrills.

That's not to say the movie isn't intense. Far from it. The end mission is one of the most well-crafted, heart-pounding sequences of the year. Just the helicopter flight into the compound gets the blood pulsing. Even the investigation, mainly people talking in board meetings and sitting at desks, attempting to put vague pieces together, is riveting to watch. Bigelow and Boal give the story a sense of urgency – a surprising amount considering "Zero Dark Thirty" is over 150 minutes – that makes every action and meeting feel like everything weighs in the balance.

Best of all, Bigelow backs off on some of the excesses that infected her previous film, namely the stylish slow motion shots of dirt rattling and bullet casings bouncing off the ground. "Zero Dark Thirty" smartly strips itself of those showy moments in favor of a cold, calculated approach that best lays out the story, the characters and the dilemmas for the viewer to parse out.

The only misstep is the use of fairly needless chapters that break up the film, but that's such a minor nitpick, it's hardly worth mentioning. It's strange but certainly not detrimental.

The result is a smart, contemplative thriller that's emotionally challenging. The ending sequences may be intense, but when the mission is done and the target is eliminated – with plenty of children and family on hand – it's hard to feel satisfied. Even the man who ends up shooting bin Laden has to be told by a fellow SEAL about what he's done.

"Zero Dark Thirty" refuses to wave flags and hand out easy victories. It's about the dark exhausting path of obsession that looks into the cost of justice in modern war. Even Alexandre Desplat's haunting score over the film's last act hints at Bernard Herrmann's famous theme from "Vertigo," another movie about an unsatisfied obsession.

By the end, I walked out of the theater exhausted – and deservedly so. An almost decade-long investigation should not feel short. There was no catharsis or sense of relief, but I still felt a rush. It was the rush provided by a director, an actor and a writer all working at the top of their game, providing plenty of tough questions and no easy answers.