As the picture begins, a 17-year-old girl has been found murdered, and top-notch Los Angeles cops Will Dormer (Al Pacino) and Hap Eckhart (Martin Donovan) have been summoned to the mountainous town of Nightmute, Alaska, to investigate. They are paired with novice Officer Ellie Burr (Hilary Swank), who idolizes Will so much that she wrote her thesis paper on one of his acclaimed cases. After cleverly setting a trap for the killer on a thickly fog-shrouded day, Will accidentally shoots the wrong man, leading to his own series of lies to cover-up his wrongdoing. In a foreign place where the sun never sets into darkness, Will further becomes sleep-deprived, leaving his own mentality confused and unsteady as the case presses forward. The plot thickens when he finally comes face-to-face with the murderer--a chillingly level-headed novelist by the name of Walter Finch (Robin William)--who threatens to expose Will's deception if he turns him in.

More than anything else, "Insomnia" proves to be a skillfully complex and engrossing character study of Will Dormer. In a multilayered, enrapturing performance that is as strong as any he's given in the last decade, Al Pacino (1999's "Any Given Sunday") has been gifted with a leading role that refuses conventional heroism over intriguing character flaws and deep-rooted hang-ups. Aided by Christopher Nolan's expertly designed direction and Dody Dorn's editing, Pacino is especially good in the later sections, when his severe lack of sleep finally catches up with him. Getting hallucinations and in a constant state of disorientation, Nolan does a superb job in allowing the audience to get into the mindset of Will Dormer.

Where "Insomnia" also excels is in its preference for intelligent storytelling and character-building moments over commonplace action scenes and exploitive violence. While the plot developments are never less than engrossing, they refreshingly come about through the interactions between the ensemble of tightly woven characters. The imaginative, eerie setting of Nightmute, lusciously photographed by cinematographer Wally Pfister, evolves into a character in and of itself. It was also nice to, for once, find a film that does not bog itself down through the insistence of making the murder investigation a whodunit mystery. When the suspect, Walter Finch, is introduced midway through, the cat-and-mouse game between himself and Will only grows more and more suspenseful from the intricacies of both parties.

Surrounding Al Pacino is a superlative cast of actors who all make an indelible impression. Foregoing his usual comedy shtick for the dead-serious, introverted, disturbingly calculated role of Walter Finch, Robin Williams (2002's "Death to Smoochy") has never been better. So penetratingly rooted in his character Williams goes that you forget after his initial appearance who exactly it is that you are watching.

As the admiring, sharp-witted Ellie Burr, Hilary Swank (2000's "The Gift") finally gets a chance to prove her 1999 Academy Award win for "Boys Don't Cry" was certainly no fluke. Swank ably stands her ground against veterans Pacino and Williams every step of the way, and gradually introduces surprising depth and character facets that you initially wouldn't expect.

They say that there are no small roles for great actors, and this is exactly what the supporting players have realized. Maura Tierney (1999's "Instinct"), as a hotel worker who forms a bond with Will; Martin Donovan (1998's "Living Out Loud"), as Will's ill-fated partner Hap Eckhart; and Jonathan Jackson (1999's "The Deep End of the Ocean"), as the victim's allegedly abusive boyfriend, Randy, do top-flight work and manage to steal their scenes.

Who impresses more than any of them, however, is Katharine Isabelle (2001's "Bones"), doing outstanding, powerful work as Tanya, the victim's best friend who shares a naked, soul-bearing moment with Will. Isabelle, one of the most promising young actresses who is slowly building up a respectable resume of work (she was also in 1998's "Disturbing Behavior" and 2000's overlooked cult film "Ginger Snaps"), has only a handful of scenes, but her portrayal is unforgettable.

Solidifying his place as an invigorating filmmaker on the rise, Christopher Nolan has taken a more linear approach to this material than he did with the backwards-in-time "Memento," but his efforts are no less impressive. Never losing its way from beginning to end, "Insomnia" is a challenging piece of work that doesn't just thrill, but provokes thought out of its attentive evocations. To be able to walk out of a thriller and know that the film you have just seen is going to prove unshakable from your memory for a long time to come scarcely occurs. That, ultimately, is what "Insomnia" achieves.