A journalist is aided by a young female hacker in his search for the killer of a woman who has been dead for forty years.
Forty years ago, Harriet Vanger disappeared from a family gathering on the island owned and inhabited by the powerful Vanger clan. Her body was never found, yet her uncle suspects murder and that the killer is a member of his own tightly knit but dysfunctional family. He employs disgraced financial journalist Mikael Blomkvist and the tattooed, ruthless computer hacker Lisbeth Salander to investigate. When the pair link Harriet's disappearance to a number of grotesque murders from almost forty years ago, they begin to unravel a dark and appalling family history; but, the Vangers are a secretive clan, and Blomkvist and Salander are about to find out just how far they are prepared to go to protect themselves.
In Stockholm, Mikael Blomkvist is a leftist journalist who's lost a libel case brought by a right-wing industrialist who set him up. While he awaits his jail sentence, a fading CEO hires him to find out who (if anyone) murdered his niece who disappeared 40 years ago. It happened on an island with few suspects, almost all members of the magnate's family. Mikael combs through files and news photos, interviews the investigator, and gets unanticipated help from Lisbeth Salander, a computer hacker with few social skills, a checkered childhood, and an abusive court-appointed guardian, the first of several men we meet who hate women. The investigation puts them in danger. Murder will out?
In Stockholm, journalist and chief-editor of the magazine Millennium, Mikael Blomkvist, is sentenced to three months in prison for slandering the corrupt entrepreneur Hans-Erik Wennerström. Wealthy industrialist Henrik Vanger and his lawyer Dirch Frode subsequently hire Milton Security to investigate the life of Mikael, whereby their ace hacker, Lisbeth Salander, finds that he is an honest man. Henrik thereby invites Mikael to Hedeby Island near Hedestadt to meet him at his home and ask him to investigate the 40-year-old disappearance of his beloved niece Harriet. Accepting the task, Mikael moves to Hedeby Island where Henrik delivers him all the files of the case. The journalist learns that all the members of the dysfunctional Vanger family are suspect and that Henrik's three brothers were all once Nazis. Meanwhile, Lisbeth, still hacking Mikael's computer (and dealing with personal problems from her new court-appointed sponsor), decides to help Mikael with further information about the case. The two pair up and soon discover a series of hideous murders connected to the disappearance of Harriet.
- Mikael Blomkvist, a journalist who works for the magazine Millennium, a local Swedish newspaper that publishes left-wing political articles, loses a libel case brought on by corrupt Swedish industrialist Hans-Erik Wennerström. The magazine & Blomkvist are ordered to pay damages and Blomkvist is sentenced to a short prison term.
While waiting to serve his sentence, Blomkvist is approached by a lawyer for Henrik Vanger, the frail & wealthy octagenarian of The Vanger Group, who lives in a remote island located several miles to the north of Stockholm, surrounded by other family members' houses.
It turns out Mr. Vanger wants to hire Blomkvist to unearth the truth behind his niece Harriet's disappearance some 40 years ago when she was sixteen. Although her body was never found, Mr. Vanger believes it was murder because that day there was an accident on the only bridge that links the island with the mainland. The bridge was closed, nobody could get out & nobody could get onto the island. Yet, Harriet just vanished into thin air. Although 40 years have passed on the cold case, Mr. Vanger still hopes to learn what happened to her, before he dies.
Before going to the island to see Mr. Vanger, Blomkvist is unaware that he is being followed & photographed by Lisbeth Salander, a Goth girl who makes a living as a computer hacker and who works part-time at Merrin Security Inc.
The young woman has a troubled past---due to crime committed in her youth, (later we learn that when she was much younger, she had set fire to a man, presumably her abusive father, in a car) she must report to a probation guardian who controls her finances. One day, she is informed that her previous guardian has had a stroke, and a new one has been assigned to her. During her first meeting with him, a lawyer named Nils Bjurman, he forces her to perform oral sex on him, threatening to accuse her of causing trouble and have her put into a psychiatric hospital, if she does not comply.
Some days later, Lisbeth Salander is attacked by punks in a subway tunnel and during the scuffle, her laptop computer is damaged, which requires her to re-visit Nils Bjurman to request some of her own money to replace it. She arrives at his apartment to get the money for the new computer, fully expecting she might have to endure further sexual harrassment from him, but is prepared, with a camcorder hidden in her purse, to record the violation. In spite of her seeming cooperation, Bjurman catches her completely off guard by throwing her down and hits her violently, then handcuffs her to his bed and binds her legs, proceeding to brutally rape her... again threatening her to keep quiet about the abuse.
Some time later, Salander shows up unannounced at Bjurman's door, and turns on him with an electric stun device. She strips him naked and binds and gags him, as he had done to her. She plays what she had recorded the last time she was there so he will understand that she has proof against him. Salander proceeds to sodomize him forcefully with a dildo she finds in his room, then using a tattoo needle, carves permanently on his chest and abdomen: "I am a sadistic pig & rapist". She threatens to reveal the evidence she has to the authorities and the media, and from now on, he has to abide by HER rules---never contact her again, not touch her finances, and release her from guardianship with glowing behavior reports in a year's time. (So as not to arouse suspicion.)
During his investigation, Blomkvist finds out his PC is being hacked into, after Lisbeth Salander sends an "anonymous" clue to him to assist him in the investigation of the disappearance of Vanger's niece, Harriet. He's able to trace back to Salander and comes to her apartment back in Stockholm where Lisbeth has just spent the night with another woman, named Miriam Wu, whom Lisbeth apparented picked up the night before. He persuades her to collaborate with him in his research. Together they dig up more and more troubling information which shows something more sinister than just Mr. Vanger's relatives preying on his fortune and who, perhaps, had succeed in getting rid of Harriet for the sake of inheritance.
As the investigation (and their relationship) progresses, Blomkvist and Salander return to the cottage Blomkvist is renting one night to discover the lock has been picked & someone has been examining their research. Blomkvist decides to pay a clandestine visit to the home of one of Mr. Vanger's brothers who had always been Nazi sympathizers. He is surprised inside, however, by the old man, with a rifle pointing to his face. Martin, Harriet's brother, appears & tries to calm down the old man, then leaves with Mikael.
Blomkvist confides to Martin a lot of information that he has dug up, and Martin leaves the room under the pretense of calling police. While Martin is out of the room, Mikael suddenly questions what Martin was doing in the old man's house, too, but the revelation comes to late, as Martin injects him in the neck with some kind of tranquilizer. When he awakens, he finds himself tied up and Martin reveals himself as a serial killer who has killed many women over the past four decades. He shows Blomkvist a collection of photos of all the dead women he raped & mutilated. The reason: "I take whatever I want". He admits that his father was a religious fanatic, and had taught him how to strangle victims. Martin puts a noose around Blomkvist's neck and winches him up to hang him.
Salander, after doing more research at the archives of Mr. Vanger's company, comes back to the house where they're staying and finds that Blomkvist is missing. After viewing the surveillance video and seeing who the intruder was, Salander rushes to Martin's house just in time to save Blomkvist from being hanged, taking a golf club to Martin's head and arms. Despite having a broken right arm, Martin gets away in his car and drives full speed but uncertain with Salander on her motobike in hot pursuit. To avoid a frontal collision with a big truck, Martin swerves off the roadside and the car overturns down a steep slope, coming to rest upside down. As Salander arrives, he begs and pleads for help, but she just looks at him and recalls setting the match to her abusive father in his car years before. Martin is burned alive when the gasoline leaks and catches fire as Salander just walks away.
Salander doesn't want to face the police so she writes to Blomkvist a note and leaves. With information supplied by Salander, Blomkvist goes to Australia to find a woman whose goes by the name Anita, who was Harriet's aunt and friend, but who was known to have died of cancer years back. He returns to see Mr. Vanger with Anita... or, the real Harriet... in tow.
It turns out Harriet was never killed by any of the Vanger family members. In fact back in 1965, she killed her own father after many years of being brutally raped by him, and by her own brother Martin. Her drunken Nazi father had chased her down to a pier where Harriet got into a boat. Harriet used an oar to knock him into the water, and keep him underwater until he stopped breathing. Everyone assumed he was drunk, fell into the water of the lake and drowned. However, she was seen by Martin and he continued to abuse her until he went away to college. One day she saw Martin had returned home and she decided to leave the island at once. With the help of her look-alike aunt Anita, she got off the island by hiding in Anita's car under a blanket & fled to Australia with Anita's passport. Now with Martin dead and learning her uncle Henrik had never stopped thinking of her, she decided to return to Sweden to reunite with him. For all those 40 years she sent him framed dried flowers every year, as she had done as a child, meant as a message that she was "out there somewhere," but he had always thought it was the killer who was sending them to taunt him.
Later, Blomkvist goes to prison to serve his three-month sentence. One day he receives a visit from Salander who brings him a lot of reading material. He finds it to be incriminating documents against Wennerström. Upon his release, Blomkvist again publishes these findings in the Millennium. Wennerström apparently commits suicide as a result. It is discovered that a large amount of money has been withdrawn from his Cayman bank account by a mysterious woman. Upon seeing a picture on TV, Blomkvist recognizes a now-blonde Salander and smiles.
In the final scene, Salander is shown getting out of a chauffered driven limousine, well-dressed and lovely, and walks away along the sidewalk of a sunny seaside resort with palm trees along the beach.