A middle-aged journalist, Mikael Blomkvist, publishes the magazine Millennium in Stockholm. In the opening courtroom drama, Blomkvist loses a libel case, having offered no defense for his publication of damaging allegations about a billionaire Swedish industrialist Hans-Erik Wennerström. He is sentenced to three months in prison. The court case and verdict brings the journalistic competence of Millennium into question. This, combined with the enmity of the powerful Wennerström exerting influence on corporate allies, quickly leads to losses in advertising revenue and reduced circulation. Facing jail time and professional disgrace, Blomkvist steps down from his position on the Board of Millennium in order to protect what reputation remains to the magazine. This occurs despite the strong objections of Erika Berger, Blomkvist's long time friend, 'occasional lover', and business partner. While Blomkvist considers the bleak prospect of jail-time and time away from the Magazine he helped to create, he is offered an unlikely freelance assignment by Henrik Vanger, the elderly former CEO of Vanger Enterprises. What Blomkvist is unaware of is that, before contacting him, Vanger commissioned a comprehensive investigation into Blomkvist's personal and professional history. The investigation is carried out by Lisbeth Salander, the title figure of the book. Salander is a curious creature. Twenty-five years of age, she is highly talented and earns a good income as a private investigator. Yet she is asocial to the extreme, emotionally closed, untrusting, and damaged to the extent that as a child she was removed from the care of her parents, declared emotionally incompetent and is a ward of the state. However, she is nothing if not resourceful. When faced with a guardian who abuses and rapes her, she takes revenge swiftly and violently.
Blomkvist visits Vanger on his estate on the tiny island of Hedeby, several hours from Stockholm. The old man draws Blomkvist in by promising not only financial reward for the assignment, but also promises that at the end of the contracted year, he will furnish Blomkvist with solid evidence that Wennerström is truly the scoundrel Blomkvist suspects him to be. On this basis, Blomkvist agrees to spend a year writing the Vanger family history as a cover for the real assignment which is to solve a "cold case"—the disappearance of Vanger's great niece Harriet some 40 years earlier, when she was sixteen. Vanger, a shrewd character, admits he is obsessed with finding out the truth of what happened to Harriet, and expresses his suspicion that Harriet was murdered by a member of the vast Vanger family, many of whom were present in Hedeby on the day of her disappearance. Each year on his birthday Harriet would make Henrik a present of pressed flowers. On his birthday every year since Harriet's murder, Vanger explains, the murderer torments him with a present of pressed flowers.
Blomkvist uproots himself from his life in Stockholm, moving to Hedeby in the middle of one of the coldest winters on record. Cold, depressed, and lonely, he begins the process of analysing the more than 40 years worth of information Henrik Vanger has obsessively compiled around the circumstances of the day Harriet disappeared. Hedeby is home to several generations of Vangers, all part owners in Vanger Enterprises. Under the pretext of researching the family history, and due to the small size of the island, Blomkvist soon becomes acquainted with the members of the extended Vanger family who are variously mad, uninterested, concerned, hostile, or aloof. Several are worried that Blomkvist is taking advantage of the obsession of a weak old man. The investigation of Harriet's disappearance approximates "the old Miss Marple closed-room scenario" with all the rich suspects marooned on the family estate on an island, "a village we grow familiar with, full of hostile locals peering out from behind their curtains". The locked-room scenario is apt because on the day that Harriet disappeared, the island was isolated from the mainland due to a road-tanker crash on the only bridge.
Blomkvist is resigned to being unlikely to solve the riddle of Harriet's murder given that the police were unable to do so, but he fulfills his contractual obligations by immersing himself in the case. Despite following endless dead leads, eventually a series of serendipitous new lines of evidence arise.
