Two Los Angeles homicide detectives are dispatched to a northern town where the sun doesn't set to investigate the methodical murder of a local teen.
In Nightmute, Alaska, seventeen year old resident Kay Connell is found murdered. As a favor to the local Nightmute police chief, two Los Angeles Robbery Homicide police detectives, Will Dormer and Hap Eckhart, are called in to assist in the investigation. Although renowned in the police world, both Dormer and Eckhart are facing some professional issues back in Los Angeles. In Nightmute, Dormer has a major case of insomnia due to a combination of the incessant midnight sun and from a secret he is carrying. This insomnia is causing him to be delusional. Something he is not dreaming about is that the murderer has contacted him, informing him all about the murder and the fact that he knows everything that is going on with Dormer. They begin a symbiotic relationship in keeping secrets for each individual's benefit. But ambitious young local detective, Ellie Burr, might piece the story together on her own.
- The film begins with an opening title and credits showing shots of white fabric of a person's clothes being crimsoned with someone's blood purposely applied by an unseen figure in a flashback to an earlier event interlaced with overhead shots of the Alaskan frontier.
Weary detective Will Dormer is seen seated in a charter plane arriving in a small town called Nightmute, Alaska alongside his younger partner, Detective Happy Eckhart. Upon leaving the plane they are met by beaming young detective Ellie Burr who is a self-confessed fan of Dormer's earlier investigations and work, much to Eckhart's obvious annoyance. Upon arriving at the station, Dormer, Burr, and Eckhart look over the body of 17 year old Kay Connell, who they were sent from Los Angelas to assist the local police in their investigation at the request of an old friend of Dormer's police Chief Nyback. Upon examining the body as well as Kay's room, Dormer deduces that the washed hair and clipped nails, as well as the dress worn by the corpse indicates that her murderer killed her and was respectful of the body. Dormer notices expensive dresses in Kay's room as well as a photo of Kay's friend Tanya, as well as Kay's boyfriend Randy. Dormer deduces that Kay had an admirer and decides to question Randy, only to be told that it was 10:00 at night- despite being bright as day.
Dormer and Eckhart visit a restaurant where they sit down to order food. Eckhart and Dormer engage in a discussion which soon becomes hostile as Eckhart reveals that back in LA, Dormer is under investigation by Internal Affairs for using questionable evidence to convict a criminal- which he may or may not have done. Eckhart tells Dormer that once the case is over, he will testify against Dormer in exchange for immunity, despite Dormer protesting that many criminals he helped bring down can be freed from their sentences if their cases were reopened. Dormer also expresses that no matter what he has done, it was for the good of other people. Echkhart sticks to his guns however and remains unshaken. Dormer leaves in a huff as the hotel manager as well as waitress, Rachel Clement, arrives, stating that he has lost his appetite.
The next day, Dormer connects with Burr and is charmed by her interests and aspirations as a detective. Dormer and Eckhart pull Randy out of class and interrogate him. Dormer notices that Tanya is concerned with the investigation. Dormer interrogates Randy who remains cold until Dormer states that he knows of Randy's abusive relationship as well as the fact that Kay was seeing someone else, who Randy confesses that Kay was secretive about that she never told him no matter how hard he beat her. Dormer and Eckhart investigate further, after Dormer encourages Burr to double check her evidence as well as very little detail, which she will later take into consideration. Dormer looks over Kay's possessions and sees that she collected novels from a certain author. Dormer continues to try reaching out to Eckhart, implying his innocence by saying, "Just because he picks up a bag doesen't mean he is a murderer" to which Eckhart remains cold and unshaken. A rivalry subtly deepens between the two.
Dormer and a team of local police lure the murderer into a hunting shack in a nearby foggy forest. The suspect fires a shot and wounds a local officer. The suspect flees into the fog as Dormer and Eckhart give chase. The two split up and are separated. Dormer notices he is being followed by the suspect and sees an armed silhouette in the fog nearby. He fires and the figure falls down, fatally wounded. Dormer rushes up to the fallen figure and discovers to his horror that he has shot Eckhart, who is confused by everything that has happened. Dormer confesses to Eckhart that it was he who shot him, which horrifies the profusely bleeding Eckhart who shrieks and struggles against Dormer despite Dormer's frantic attempts to stem the bleeding of his wounds. Eckhart dies painfully as Dormer tries to help him before the police arrive. A distraught Dormer tells the authorities that it was the killer who shot Eckhart, keeping from them the true information, knowing that Internal Affairs will never believe it was an accident. Dormer does not tell the authorities that he carries with him a .38 caliber pistol. Burr gets moved onto Eckhart's murder case, much to Dormer's increased nervousness. Dormer calls up Eckhart's wife Trish and relays to her the news of her husband's death. Trish breaks down and orders Dormer to kill the murderer of her husband and to not arrest him.
Suffering from hi inability t sleep due to the never-ending brightness of the sky, as well as guilt over killing Eckhart, Dormer however keeps his cool when confronting the wounded officer. He tells the officer that the killer of Kay Connell is the only one to blame for the trouble that has happened. Dormer is haunted by visions of a disappointed Eckhart watching him from a distance as Burr's team investigates the area, finding a .38 caliber bullet in the rocks. Dormer returns to the hotel where Rachel gives him a phone, telling him that another detective is on the line. Dormer answers and discovers it is another investigator who will testify against Dormer once he returns to L.A who states he would like to see Dormer's report on Eckhart's death. Worn down by physical tiredness, stress, and guilt, as well as nervousness of being discovered, Dormer verbally assaults the caller as Rachel looks on in concern.
Dormer sees a carcass of a dog in an alleyway and shoots it with his .38. He retrieves the bullet and cleans it, bringing it to the morgue with him where he sees Eckhart being covered by the medical examiner. He receives the 9 mm bullet that killed Eckhart and swaps it with his .38 used bullet. Suffering from his insomnia, Dormer visits the police station has struggles with overhearing loud noises. Burr brings to him a report closing Eckhart's death case which Dormer is tempted to sign and finalize. However, listening to his conscience, Dormer refuses and encourages Burr to recheck her evidence telling her that Eckhart's death is not to be taken lightly. He returns to his hotel room and attempts to sleep once more by taping a curtain over his windows. However, the bright sun keeps him awake once again. Burr goes over her evidence and discovers that there were facts she had overlooked regarding Eckhart's death. Eckhart attempts to sleep but is haunted by memories of tampering with evidence and killing Eckhart. As he is finally about to fall asleep, a phone call awakens him. "Can't sleep Will?" says a chilling voice at the end of the line. The caller reveals that he saw Will Dormer kill his partner, and dropped his .38- which Dormer picked up. Dormer tries to keep the man on the line but the man hangs up stating, "We're partners on this."
The next day at Kay's funeral, Dormer notices Tanya and Randy together. He offers Tanya a ride home in his truck, to which she agrees. As they are driving, Dormer attempts to question Tanya, only for her to try and seduce him. However, he frightens her into compliance by driving on the wrong side of the highway, nearly getting totaled by an oncoming semi truck. He brings her to a dump where Kay's body was found and Tanya weeps, confessing that at the time Kay was killed, Randy was cheating on her with Tanya and that because of it, Tanya will not give Randy an alibi. Dormer discovers on his own that Kay was visiting her favorite Crime author, Walter Finch who lives nearby in the vicinity. Dormer receive more calls from the mysterious man on the phone and deduces that the caller is the killer, Finch. Dormer finds Finch's apartment in the town of Umkumiut. After appeasing Finch's barking dogs, Dormer waits for Finch to arrive but Finch arrives and discovers the police are present, and flees. Dormer gives chase and Finch runs across a river waterlogged with cut timber. Dormer falls through the logs while pursuing Finch and almost drowns. He escapes and returns to Finch's apartment, having nowhere else to really go.
While waiting, the phone rings and Finch encourages Dormer to make himself at home to his shower and his bed. Dormer answers and they agree to meet each other on a ferry outside of Nightmute to discuss what to do of the present issue as well as instructions to feed his dogs as they are on a diet. Finch never returns and a defeated Dormer returns to the police department, but not before hiding Finch's .38 pistol in Finch's vent to frame him. Burr tells Dormer that she has deduced that Finch is a suspect- to which he pretends to be impressed by. Dormer enters the ferry where he meets Finch who says, "I'm not who you think I am." Finch explains that ever since he was a little boy he was fascinated by cops when his grandmother was met by one after her purse was stolen. Finch tries to appeal to Dormer saying that Kay's death was all an accident, only for Dormer to not buy into his pleas. However, Finch explains that Dormer has blood on his hands as well, having killed his partner- explaining that the two have committed accidents. Finch appeals to Dormer's original beliefs that his job and actions have made people safe, should the truth of Dormer's killing of Eckhart gets out, all of Dormer's work will be for nothing.
Dormer then agrees temporarily to Finch's terms and warns him of the interrogation coming up. Finch proposes they shift the blame of Kay's murder on Randy, and states that they need a "Wild Card," something that will be a pressure on anything that gets in the way of their plans. As the ferry makes its stop, Finch steps out and flashes a tape recorder at Dormer- revealing that he had recorded their entire conversation. "Wild Card!" he shouts, as Dormer stares on in weary defeat. Dormer returns to his hotel room where he has a vision that he saw Eckhart clearly and shot him on purpose. Finch calls up to discuss their further machinations. Dormer insists that should they trust each other, Dormer will need to know the truth of how Finch killed Kay. Finch reluctantly agrees and tells him that Kay appeared at Finch's home, distraught and drunk, explaining of how Tanya and Randy were "carrying on." Finch tried to calm her, and lost a hold of himself and kissed her. Kay laughed at him and, with his pride hurt, Finch lashed out, striking her. Seeing her look of betrayal and fearing for his relationship with her, her lost self control and beat her to death, knowing "there was no turning back." Finch breaks down slightly and cries a little before coming to a form of peace from his confession, telling Dormer, "I will be able to sleep now." Finch offers to hear Dormer's confession of killing Eckhart, but Dormer hangs up, knowing he must keep his wits about him.
Dormer meets with Burr, who notices his degradation and tells him in an effort to boost his morale, "A good cop can't sleep because he's missing a piece of the puzzle, a bad cop can't sleep because his conscience won't let him." Finch is interrogated at police headquarters and it goes smoothly until Dormer steps in and loses his temper, finding himself unable to agree to Finch's plans to frame Randy. During the interrogation, Fuinch pushes the blame upon Randy, stating that Randy had a .38 pistol, which he kept hidden "in a heating vent" subtly revealing to Dormer that he knows Dormer was attempting to frame Finch. Dormer accuses Finch of having sexual feelings towards Kay and verbally assaults him before lashing out. Dormer is removed from the room and rushes over to Randy's home, where he attempts to uncover the pistol to clear Randy's name before the search party arrives. He scours Randy's room but is fruitless in his search for the weapon. The search party arrives and finds the .38 caliber gun in a pail of motor oil- only seconds before almost finding Dormer. Randy arrives and is swiftly arrested.
Dormer meets with Finch and Finch explains that Dormer was trying reveal Finch's machinations to the cops ever since pretending to work with him. Dormer takes Finch's tape away and destroys it, before assaulting Finch, threatening to kill him. Finch refuses to take responsibility for killing Kay and Dormer asks if killing Finch would be an accident. Finch states, "If you want it to be." Dormer's conscience wins over and he leaves Finch alone, only for Finch to explain that the tape he destroyed was the only physical evidence that proves Finch's guilt. The two leave and Dormer sinks back in weariness. Burr searches the surrounding woods where Eckhart was killed and finds a 9 mm shell casing- which conflicts with the .38 caliber bullet Dormer planted in evidence. She asks Dormer for his opinion, only for him to state that the case is closed, much to her rising suspicion. As Dormer prepares to leave, Burr embraces him in farewell, and feels that he has a 9 mm. gun concealed in his clothes. Finch invites Burr to his lake house where he says he has evidence of Randy's abuse of Kay via letters she wrote to Finch, fearing for her life.
A careless and distraught Dormer returns to his hotel room, where he is haunted by flashes of memories of him planting false evidence to condemn a killer as well as the ghost of Eckhart who eyes him in disapproval. Dormer arranges his luggage to leave and tries one last time to get some sleep, only for the bright sun to keep shining through his window. Having had enough, Dormer rips apart cushions and furniture and stacks a barricade of articles against the window, only for Rachel to arrive and ask him what he is doing. Dormer explains that it is too bright in there to which Rachel explains that it is in fact dark in his room, before turning on the electric lights, which nearly blind Dormer, who asks to to turn the lights off, to which she complies.
Rachel rearranges the room as a weary and defeated Dormer sits on the bed and explains that a pedophile named Wayne Dobbes kidnapped a young boy and held him captive in his apartment where he "made him do things" before hanging the boy to death after 3 days of molestation. The case was about to be thrown out as through coincidence there was not enough evidence to convict Dobbes. Dormer explains that the judge could not understand a killer like Dormer could, and that justice needed to be served. Dormer therefore fabricated evidence by spreading blood samples on Dobbe's apartment, revealing the truth behind the scene at the beginning of the crimes. Dobbes was then convicted and killed for his crimes and since then, Dormer has been under investigation by Internal Affairs for his actions. Rachel listens in silence, looking at Dormer with concern. Dormer concludes that Hap Eckhart spearheaded the investigation against Dormer. Dormer concludes his story saying "the end justifies the means." Upon hearing Dormer's confession, Rachel states that she is in no position to judge him and explains that she came to Alaska to escape her life earlier. Dormer begs for her opinion of his actions and she replies, "I guess it's about what you thought was right at the time, and what you're willing to live with." Dormer hears this, and comes to a state of peace and understanding. Dormer washes his hands and looks at himself in the mirror, leaving his room, pausing once to look at Rachel napping peacefully on the bed.
Burr goes to Finch's lake house at Lake Kgun where Finch has not yet fabricated the letters he claimed to have received and asks fearfully. He deduces that she has realized Dormer's hand in the death of Eckhart. He knocks her unconscious. A relapsing Dormer drives to Lake Kgun, almost becoming hypnotized by the windshield wipers and almost crashing his car. Dormer arrives and Finch explains that Burr knows Dormer killed Eckhart and proposes they do something about it. Not wanting to be roped into another murder, Dormer holds Finch at gunpoint as Burr comes to. Dormer suffers from hallucinations from lack of sleep, and Finch takes advantage of this, beating him to the floor savagely. Burr intervenes and Finch retreats to his fishing shack. Burr holds Dormer at gunpoint, stating she knows he killed Eckhart. Dormer confesses that he did so, and it was an accident, as he could not see Eckhart through the fog. However, he confesses that he is now unsure of whether it was an accident or not. Burr believes he is innocent, but then the two are attacked from the fishing shack by Finch. Burr distracts Finch by exchanging gunfire as Dormer sneaks into the shack from behind. After a brief scuffle, Finch and Dormer exchange weapons in a confusing debacle. "Forget the Wild Card," Finch states as he aims Dormer's 9 mm at Dormer. The two shoot each other at the same time, the shotgun blowing Finch's stomach open as Dormer is shot in the chest by his own gun.
Finch looks down at his fatal wound before falling lifelessly into the frigid waters of the lake under the fishing hole. Dormer watches Finch's dead body sink below the frigid waters of the lake into the darkness. Dormer staggers out onto the dock where Burr arrives and tries to help him. Knowing that he is dying, she takes out the 9 mm casing, saying that she knows Dormer did not mean to shoot Eckhart and that "Nobody has to know." She prepares to throw the casing into the lake but Dormer stops her, insisting that the world needs to know the truth and that Burr do the right thing, instructing her, "Don't lose your way." Dormer dies peacefully in Ellie Burr's arms, saying quietly, "Let me sleep."
The film ends with Ellie Burr standing on the dock, after placing the casing back in the evidence bag. She looks ahead determined, looking down with reverence at her dead hero who died preventing her from making the mistakes of doing the wrong things for the right reasons as he had done.
After the end credits, the film's title "Insomnia" appears once more before fading to darkness.