Mikael Blomkvist is an intrepid investigative reporter in Stockholm, Sweden, who has been convicted of libel against Hans-Erik Wennerstrom, a powerful business mogul. Following his trial, Blomkvist is determined to clear his name. After getting a mysterious phone call from a lawyer representing Henrik Vanger, the well-respected retired CEO of Vanger Corporation, Blomkvist meets with Vanger and learns of the mysterious disappearance 40 years earlier of Vanger's 16-year-old grand-niece, Harriet Vanger. Blomkvist accepts the business proposition that Vanger offers him: In return for trying to solve the mystery surrounding Harriet's disappearance, Vanger will provide Blomkvist generous financial compensation in addition to proof that Wennerstrom is a crook.
While Blomkvist becomes involved with the dysfunctional Vanger clan, we meet Lisbeth Salander. Salander is a 24-year-old hacker with a photographic memory and a dragon tattoo. Salander's troubled childhood and her refusal to cooperate with mental health professionals caused her to be declared mentally incompetent as a youth. However, with the help of Holger Palmgren, her trusted guardian, she's able to get a job as a private investigator for Milton Securities, for which she investigates both Blomkvist and Wennerstrom. Salander's stable life is threatened when Palmgren suffers a stroke and she comes under the guardianship of Nils Bjurman, who takes advantage of his power over her and rapes her repeatedly. After videotaping one of the rapes, Salander blackmails Bjurman into letting her regain control over her own finances and her life. A few weeks later, she's offered the chance to join Blomkvist in his investigation. Salander eagerly accepts the offer as she finds herself drawn to Blomkvist and sympathetic to elements of Harriet's case.
As Salander and Blomkvist grow to trust each other, they develop a tenuous romantic relationship. Together, they are able to discover that Harriet was not killed as Henrik Vanger suspects. Salander and Blomkvist learn that Harriet had connected the gruesome murders of women across Sweden in the 1940s-1960s to verses from Leviticus. Exploring this connection, Salander and Blomkvist are able to connect the murders to Harriet's deceased father, Gottfried Vanger, and his son, Martin Vanger, the current CEO of Vanger Corporation. Salander and Blomkvist are then able to figure out that Harriet was not murdered, but fled the family to escape Martin, who, like their father, violently raped her. Once Blomkvist and Salander discover Martin's wrongdoings, they have a violent confrontation with Martin, resulting in his suicide. Following this altercation, they find Harriet in Australia and reunite her with a surprised and thankful Henrik Vanger.
Having solved the mystery, Blomkvist remains disappointed that the information Henrik provides on Wennerstrom is over 30 years old. Salander comes to his rescue and reveals that she's copied Wennerstrom's entire computer onto a server during her initial investigation of him. Delighted, Blomkvist enters several weeks of intense writing as he creates not only an article for the magazine he co-owns, but also a book-length report on Wennerstrom's corrupt dealings. Salander, also interested in Wennerstrom's wrongdoings, particularly against his former girlfriend, seeks revenge on Wennerstrom. After studying the structure of his financial empire, Salander is able to skillfully siphon millions of dollars from his accounts and place the money into anonymous accounts to which only she has access.
Blomkvist is thrilled with his successes and is interviewed by countless media outlets about his investigation of Wennerstrom. Salander slowly realizes that she no longer sees Blomkvist as just a friend — she's fallen in love with him. This revelation threatens the self-contained universe Salander has created for herself, making her feel vulnerable. She decides to share her feelings with Blomkvist. But, when Salander sees Blomkvist enter his apartment with another woman, she chickens out and returns to her own apartment, bottling her feelings once again.