The film picks up right after The Girl Who Played with Fire, featuring the brilliant computer hacker, Lisbeth Salander, who is airlifted to a hospital in Gothenburg to recuperate from severe gunshot wounds inflicted by her father, crime lord Alexander Zalachenko. Meanwhile, journalist Mikael Blomkvist, who previously benefitted from Salander's expertise, is determined to exonerate her of multiple murder charges. He knows well that Salander was set up by a covert group within the Swedish Security Service known as the "Section," which provided shelter to Zalachenko after his defection from the Soviet Union.
As tension mounts, Section operatives Evert Gullberg and Fredrik Clinton conspire to ensure that both Zalachenko and Salander are silenced to protect their dark secrets. The situation escalates when Gullberg arrives at the hospital and kills Zalachenko but fails to get to Salander and ultimately takes his own life. Clinton's next move involves having Salander forcibly committed back to the mental institution, a chilling location where she was unjustly held as a child due to her traumatic past involving Zalachenko. Playing a pivotal role in this sinister plan is Dr. Peter Teleborian, the institution's head, who had subjected young Salander to brutal treatments.
In a twist of fate, Blomkvist manages to convince Salander's doctor to sneak her an Internet phone, leading her to reach out to her hacker ally, Plague. Together, they investigate Teleborian's past as Salander prepares to expose heinous allegations against her state-appointed guardian, Nils Bjurman, a former Section operative who allegedly raped her.
Amidst the backdrop of a murder trial, the storyline intensifies with the presence of Ronald Niedermann, Zalachenko's menacing son and assassin, who remains at large after murdering a police officer. One member of an outlaw motorcycle gang, Sonny, soon learns of Niedermann's whereabouts and uncovers a tragic scene in which his brother is dead and girlfriend injured. Fueled by revenge, he sets out to find Niedermann.
On the day of her trial, Salander makes a striking entrance, adorned with piercings, a mohawk, and all-black attire. Called as an expert witness, Teleborian’s portrayal of Salander as unstable is gradually dismantled by her lawyer, Annika Giannini, who uses compelling evidence including a video that substantiates Salander's claims of assault against Bjurman.
As Giannini presents her case, law enforcement conducts a crucial operation to arrest key members of the Section, ultimately confirming that the evidence against Salander was fabricated. Teleborian's deep-seated corruption is exposed when Plague hacks into his laptop, revealing incriminating child pornography, leading to his arrest and Salander's release.
In the final act, Salander visits a property inherited from her father, where she discovers Niedermann hiding out. A harrowing confrontation ensues, resulting in her using a nail gun to incapacitate him. After alerting the police and contacting Sonny for assistance, Salander watches as the police arrest Niedermann, who is later killed by the motorcycle gang.
In a poignant moment, Blomkvist meets with Salander to inform her of Niedermann's fate and the arrests made thereafter. Their paths diverge once more as they share a moment of reconciliation, each embarking on their respective journeys as the film draws to a close.
