Al Pacino delivers one of the best performances of his career as L.A. detective Will Dormer. When we first meet Dormer, he and partner Hap Eckhart (Martin Donovan) are being flown to a small Alaskan town to help investigate the brutal murder of a 17-year old girl. They are teamed with Ellie Burr (Hilary Swank), a local cop who has been an admirer of Dormer's work for years. Their investigation brings up predictable leads including the girl's abusive boyfriend. It seems like a cut-and-dried case that should send the detectives home quickly. But then we learn something about Dormer and Hap. Part of the reason they are in Alaska is that their department is being investigated by Internal Affairs. Hap is considering cooperating in the investigation, but Dormer feels this course of action could make him a target as well. He tries to persuade Hap to keep his mouth closed.

Al Pacino and Robin Williams come face to face in Christopher Nolan's new thriller Insomnia

Then things get even more complicated. The killer is lured to a small shack in the middle of nowhere. Dormer gives chase but becomes disoriented in the thick Alaskan fog and accidentally shoots an innocent cop. Not knowing what to do, he pins it on the killer, even going so far as to tamper evidence. The horror of his actions cause Dormer to lose sleep, a problem compounded by the fact that it is that particular time of the year in Alaska when the sun does not set for days. One sleepless night, he gets a phone call from Walter Finch (Robin Williams), a mystery writer who knows some things about Dormer that, if revealed, would easily shatter the illusion that anyone else killed the cop. What he wants in return is a little sympathy, for he too holds a secret that could prove life-shattering. Meanwhile, Ellie is assigned to investigate the cop's death, and her leg work starts to hit uncomfortably close to home. Over the course of the next six endless days, Dormer struggles to solve the murder case without revealing himself. All of this must be done while he fights off hallucinations and delusions caused by the constant daylight.

Most thrillers use darkness to achieve atmosphere, and the brilliance of Nolan's direction is that he uses light to achieve it. The scene in which Dormer becomes lost in the fog is a perfect example. We're so used to shadows and nightfall that it seems unusual to see someone get confused in brightness. Nolan stages the scene effectively, pouring the fog on so thick that we, too, become disoriented. He also makes the most out of the locations (actually, the movie was shot in Canada), which are breathtaking in their utter largeness. The scenery - with its sweeping waterfalls, towering mountains, and isolated beauty - is a character in and of itself. You don't see this kind of setting in many movies, which gives it an added punch. Take, for instance, a chase through a logging company. Dormer jumps from one log to another as they float along the pier. He falls through, but his attempts to climb back out are foiled when the logs start smashing violently into one another. It's a literal heart-pounding moment, a perfect case of the director using the natural attractions of the location to create a very realistic sense of danger.

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Nolan and his cast may get some thrills out of the material, but the focus is squarely on the moral dilemmas that are everywhere. The screenplay (beautifully written by Hillary Seitz) uses Dormer's sleep deprivation as a metaphor for the mental anguish that would doubtlessly keep him awake anyway. He wonders: Do the ends really justify the means? Is it okay to lie or tamper with evidence so long as it sends a bad guy to jail where he belongs? In this role, Pacino has found gold. The actor allows himself to look more and more disheveled as the story progresses. You can see him wearing down, and as much as I admired Pacino's performance, I ultimately forgot it was a performance. He really inhabits this role in a way that deserves serious Oscar consideration.

I have been careful not to give away too much about Williams's character, although this is clearly a different kind of part for him. Walter Finch, like Dormer, is ambiguous. You could make a case that he's a good person trapped by a bad situation just as easily as you could argue that he's fatally flawed. Williams and Pacino have a number of amazing scenes in which their characters taunt each other with what they know. At the same time, both are aware that screwing the other guy would essentially mean screwing themselves. This is powerful stuff - a meeting of the troubled minds. In the middle of it is Hilary Swank, who balances everything out by acting as the ethical center.

The beauty of Insomnia is that it's not about who-dunnit or why they dunnit. It's about the choices people make, the way one bad choice can jam you up for a lifetime. As I watched the film, I realized that it was unsettling me - not because of the violence (which is minimal) but because Nolan and his cast do such a superb job of exploring the deeper themes of the plot. Not only is Insomnia one of the most intense police procedurals ever filmed, it's also one of the most riveting moral dramas I've ever seen.

( out of four)

Insomnia is rated R for language, some violence, and brief nudity. The running time is 1 hour and 58 minutes.