The Girl Who Kicked The Hornet's Nest begins the day after The Girl Who Played With Fire ends. (Check out our "Brief Summary" of The Girl Who Played With Fire.) Mikael Blomkvist is arrested by a bumbling policeman at Alexander Zalachenko's farm. But not before he patches Lisbeth Salander's wounds with duct tape and has her and her father (that nasty piece of work, Zalachenko) flown to Sahlgrenska hospital in Göteborg, Sweden. At the hospital, where Salander is under arrest, Dr. Jonasson removes Zalachenko's bullet from Salander's brain. Zalachenko is treated for his axe wound to the face (courtesy of Salander). They are recovering two doors down from each other.
Meanwhile, Blomkvist gets out from under arrest, and the cops begin to realize that Ronald Niedermann, son of Zalachenko and half-brother of Salander, is guilty of the murders they suspected Salander of committing. He's also guilty of many others. Niedermann escapes by killing one policeman, wounding another, and kidnapping a woman named Anita Kaspersson. He disappears.
Blomkvist convinces his sister, Annika Giannini, to be Salander's lawyer, if Salander wants her to be. When Salander regains consciousness, she's not a happy camper. Mostly, she wants to kill Zalachenko once and for all.
Back in Stockholm, Blomkvist starts writing Salander's story. Soon he learns that Erika Berger, his long-time lover and editor in chief of Millennium magazine, is going to work at SMP, a big news organization with a board of directors that's notoriously hostile to women. Still, he keeps up his work on Salander's case, including visiting Milton Security to make sure that Dragan Armansky, Salander's former boss, is on board to help Salander.
Evert Gullberg comes on to the scene. He, like Zalachenko, is in his seventies. He remembers how he came to be the head of a secret group inside the Security Police, known as the Section. The Section is devoted to protecting Zalachenko, an ex-Russian spy turned (now retired) Swedish informer. The Section is responsible for Salander (now almost 27) being committed to St. Stefan's children's psychiatric hospital in 1991, where she was subsequently declared incompetent.
While Gullberg muses, Zalachenko and Salander plot to kill each other in the hospital. Later, Gullberg meets up with other members of the Section. They hatch a plot to have Salander committed to an insane asylum for life.
Blomkvist begins forming Team Salander and planning strategies to free her. He realizes that his phones are bugged and documents for Salander's case have been stolen. Salander accepts Giannini as her lawyer. While Salander and Giannini are conferring, Gullberg sneaks into Zalachenko's room, shoots him dead, and then kills himself.
Things are getting more serious. Blomkvist notices that he's constantly being followed by men in a gray Volvo. Still, he continues writing his book about Salander, Zalachenko, and the Section (though he doesn't yet know it's the Section). He plans to publish it during Salander's trial.
Back in the hospital, Salander wants everybody (especially the police and her lawyer) to stop bugging her and asking her questions. Peter Teleborian, the sadistic psychiatrist who tormented Salander at St. Stefan's when she was a child, tries to convince her doctor, Dr. Jonasson, to let him see Salander. Jonasson refuses; he doesn't believe Salander is insane. Jonasson is really taking an interest in Salander, and Blomkvist convinces him to smuggle Salander her Palm Tungsten T3. When Salander gets her Palm, she discovers she's connected to the Internet. She immediately links up with her friends in the Hacker Republic, and they agree to help with her case.
Meanwhile, Erika Berger has started work at SMP. She tries in vain to get the board to take her ideas seriously. She also gets a nasty cyber-stalker. At Millennium, reporter Henry Cortez accidentally discovers that Berger's boss at SMP sells toilets made by child labor in Vietnam. Blomkvist gives Berger the bad news about her boss. First she gets mad at him, but then she takes him to bed.
At the same time, the police are investigating Salander's case. Torsten Edklinth and Monika Figuerola of the Constitutional Protection Unit of the Security Police join Team Salander.
All of our favorite characters are working on freeing Salander. Blomkvist writes and researches at Salander's huge, top-secret apartment, since his place is bugged and crawling with secret agents. Salander starts writing her life story on her Palm. Plague, Salander's hacker friend, finds thousands of violent, sexual images involving children on Teleborian's computer. They're beginning to build a case to discredit the evil psychiatrist who's determined to keep Salander imprisoned.
Berger's stalker escalates, breaking into her home and stealing private items like sex tapes and love letters. Milton Security gives Berger a security system overhaul.
Figuerola and Blomkvist work on Salander's case together, and soon Figuerola seduces him (rather easily).
Salander finishes her autobiography and sends it to Blomkvist. She has recovered from her brain surgery and is about to be moved to a jail in Stockholm to await her trial. While waiting to be moved, though, Salander does a bit of hacking and realizes Berger has a stalker. She identifies him as a news guy at SMP, Peter Fredriksson. Suzanne Linder of Milton Security confronts Fredriksson and gets back Berger's personal items.
At SMP the next day, Berger's boss finds out that Millennium is going to expose his child-labor toilet business, and accuses Berger of being behind it. She resigns from SMP, but first has Cortez's toilet article about her boss published on the front page of SMP. Everybody at Millennium is glad to see her back.
Blomkvist's book about Salander and the Section is to be published the same day as her trial. The Section wants to prevent the piece from being printed, but when they reach the publishing house they find that Milton Security is guarding Blomkvist's book. They decide to have Blomkvist killed and plant cocaine and a wad of cash in his apartment, so his death looks drug-related.
Blomkvist's home security cameras record the Section planting drugs in his place. At dinner with Berger, hit men hired by the Section try to kill Blomkvist, but he and members of Team Salander foil the killers.
Salander's trial takes place. Teleborian is the chief witness, but Giannini and Salander totally discredit him. In order to build Salander's credibility, Giannini shows the court the video of Salander being raped by her ex-guardian, Nils Bjurman (see Dragon Tattoo for more on that). This proves Salander isn't delusional and lying about the rape, as Teleborian has claimed. At the end of the trial, members of the Section are arrested for a slew of crimes. Teleborian is also arrested for having violent child pornography on his computer. Salander's declaration of incompetence is revoked, and all charges are dropped against her.
Back in Stockholm, Salander visits an abandoned factory she inherited from Zalachenko. She finds a body there and quickly realizes the place has been used to keep women against their wills. She also realizes she's not alone. Her half-brother, Ronald Niedermann, has been hiding out here for most of the year. Niedermann (a gigantic man who feels no pain) wants to kill Salander. When they face off, though, she manages to nail his feet to the floor with a nail gun. She contemplates killing him but decides against it. Instead, she lures some of his enemies (also criminals) over the factory, then calls the cops on everybody.
Back at Salander's place, she hears a knock on the door. It's Blomkvist, bearing coffee and bagels. He wants to be friends with her again. She "open[s] the door wide and let[s] him back into her life" (Epilogue.261).