Spiders and quantum computers......

Plot[edit]

Frans Balder, a computer scientist, returns to Sweden—abandoning a prestigious job with a Silicon Valley company—to take custody of his autistic son August. Balder is informed by several law-enforcement agencies that he is in danger from a criminal organization who call themselves the "Spider Society", but he ignores their warnings, preferring to focus on his neglected son. August exhibits savant syndrome; he produces drawings of impressive veracity and demonstrates facility with numbers.

Mikael Blomkvist feels like he is stuck in a rut. In the year since Millenium magazine's scoop on "The Section" (as chronicled in The Girl Who Kicked the Hornets' Nest (2007)), the publication has stagnated and is in danger of losing creative control to the whims of outside investors. A former associate, Linus Brandell, puts Blomkvist on Balder's scent and relates Balder's tumultuous history. Blomkvist is largely bored with the recitation until Brandell mentions that some of Balder's activities were aided by Lisbeth Salander.

Spurred by a childhood memory, Salander is attempting to track down someone from her past, and her investigations have led her to the Spider Society. She has taken on a side project: helping the Hacker Republic gain access into the servers of the US National Security Agency. As the novel opens, she succeeds, much to the fury of the agency's top IT Security agent, Ed "The Ned" Needham. This puts her on the NSA's radar, making her a top priority alongside other persons of interest whom NSA agent Alona Casales and agent Gabriella Grane of the Swedish National Defence Radio Establishment are working on identifying: an elite group of Russian criminals who call themselves the "Spider Society." They are led by an individual named "Thanos".

Grane calls Balder with concerns about his safety, and Balder hires Milton Security for protection. He also reaches out to Blomkvist, hoping to confess his concerns to a respected journalist. Blomkvist agrees to meet him, but as he arrives, an assassin, self-identified in the narration as "Jan Holtser", kills Balder. Blomkvist reaches out to Salander, hoping to harness her talents to the investigation.

August is returned to his mother's custody, but Hanna is unable to cope with his disorder. Eventually, unable to stop him from drawing a picture of a checkerboard pattern, Hanna remands August to a psychological care facility. However, Blomkvist recognizes that the picture, if completed, would depict Jan Holtser mid-murder, and he enlists the help of the police to find August a safe place to unburden himself of the image. August's status as a key witness makes it to Holtser's superior, a woman called Kira, and she orders Holtser to eliminate the child. August is saved by Salander, who escapes with him. Salander reaches out to Blomkvist and Millenium editor Erika Berger for help; Berger then contacts Grane, who offers her beachfront vacation property as a safe house.

Blomkvist reaches out to Balder's former associates and learns that Balder hired Salander to confirm that someone had robbed him, implicating executives inside Solifon in the theft. Thus, Balder went to the company to attempt to gain more evidence. He discovered that Solifon was collaborating not only with the NSA in their espionage efforts, but with the Spider Society, as well. It is this investigation, not Balder's groundbreaking work on quantum computing, that resulted in his death.

In America, Needham is pulled off the investigation into the agency's hack; NSA top brass do not wish it exposed that they have engaged in industrial espionage for profit. Needham has not had much success in tracking down his nemesis, but his employees have asked around for who might have been able to perpetrate it. The name that keeps coming up is "Wasp". Needham draws the parallel between the Marvel Comics superhero Janet van Dyne, founding member of the Avengers, and The Spider Society, the fictional organization who oppose "The Sisterhood of the Wasp". He also draws the connection to Thanos, a supervillain faced by the Avengers.

Blomkvist is approached by a woman calling herself Rebecka Mattson, who attempts to seduce him. Blomkvist, however, sees through her act. He contacts Salander's former guardian, Holger Palmgren, and confesses that he recognized the woman: Camilla Salander, Lisbeth's long-missing twin sister. Palmgren tells Blomkvist of Camilla's nature: attractive, popular, predatory, manipulative, and intensely interested in her father Zalachenko's approval, and he describes how Camilla, noticing Lisbeth's love of comic books, began styling herself and her friends as van Dyne's nemeses solely to antagonize Lisbeth. Blomkvist and Palmgren theorise that Camilla has taken over part of Zala's former criminal network and is using it to strike against Lisbeth.

Blomkvist is contacted by Needham, who has come to Sweden to reach him. Needham gives him details of Salander's hack and asks to meet with Salander, so that she can help him secure the NSA database. However, they are interrupted by Salander: she and August are under attack at their hiding place by the Spider Society. Lisbeth fends them off, leaving several suspects, including Holtser, wounded on-site. Though Camilla gets away, Lisbeth and August do as well; additionally, August has already drawn the picture that will condemn Holtser and has helped Salander decrypt the last of the NSA's secrets.

Gabriella Grane takes a position at the UN working for human rights. Millenium publishes an exposė of the affair which restores their credibility; additionally, an new investment from Gibraltar (the site of Salander's Wasp Enterprises) allows Millennium to buy out their meddlesome investors. Needham, with the magazine as ammunition, ousts the crooked officers at the NSA.

Salander visits Blomkvist to renew their friendship.