In Stockholm, journalist Mikael Blomkvist finds himself grappling with the repercussions of a libel case initiated by businessman Hans-Erik Wennerström. Seeking redemption, he is approached by the affluent Henrik Vanger who offers vital evidence against Wennerström, but with a peculiar stipulation: to investigate the unresolved disappearance and suspected murder of his grandniece, the 16-year-old Harriet, which occurred four decades ago.
Each year, Henrik receives a framed flower, reminiscent of the ones Harriet used to send him on his birthday, leading him to believe that her murderer might be tormenting him. To delve deeper into the case, Blomkvist relocates to a small cottage on the sprawling Vanger family estate located on Hedestad Island, initiating his investigation into Harriet's past.
Enter Lisbeth Salander, an introverted investigator and skilled hacker, who is trapped under the tyrannical rule of her state-appointed guardian, Nils Bjurman. Bjurman exploits his power over Lisbeth, manipulating her finances and coercing sexual favors through threats of institutionalization. Their encounters take a horrific turn when he chains her down, and while Bjurman underestimates her, Lisbeth turns the tables by recording his assault. During a subsequent meeting, she retaliates fiercely: after subduing him with a taser, she extracts revenge, binding him and marking him with a tattoo that reads "I’m a rapist pig." This act empowers her to blackmail him into granting her financial freedom and ending their interactions.
As Blomkvist scrutinizes the enigmatic island, he interviews members of the Vanger family, uncovering unsettling links to Nazi affiliations during World War II. Among various findings, he comes across a list that corresponds to young women who were brutalized between 1947 and 1967. Partnering with Lisbeth, they uncover that many of these victims bore Jewish names, suggesting that antisemitism might have been a driving force behind the murders. Disturbingly, Blomkvist receives an ominous omen: he finds his cat's dismembered body at his doorstep, and narrowly escapes a near-fatal bullet graze, leading to an intimate moment between him and Lisbeth.
Their journey of discovery leads them to Harriet's family ties, pointing towards her father, Gottfried, and later her brother, Martin, for the heinous crimes. In one fateful encounter, Blomkvist is captured by Martin and locked away in a grotesque basement where Martin revels in his past atrocities of killing and sexually assaulting women. Remarkably, just as Blomkvist faces his end, Lisbeth arrives, injuring Martin and forcing his escape. In a tense pursuit, Martin's reckless driving results in a catastrophic accident involving a propane tank that leads to his demise.
As Lisbeth aids in Blomkvist's recovery, she reveals a dark fragment of her past, recounting how she had been institutionalized after setting her father ablaze as a child. Their investigation leads them to conclude that Harriet is still alive and hiding. Upon tracking her down to London, Harriet shares her horrifying ordeal of abuse and manipulation at the hands of her father and brother. Finding refuge through her cousin Anita, who later perished in an accident, Harriet bravely returns to Sweden, reuniting with Henrik in a heartfelt moment.
Though Henrik provides Blomkvist with the anticipated incriminating details against Wennerström, they soon realize the information is too old to hold any legal weight. Meanwhile, Lisbeth ingeniously hacks into Wennerström’s accounts, revealing his involvement in laundering vast amounts of money for criminal organizations. Armed with this newfound evidence, Blomkvist publishes an explosive editorial that devastates Wennerström’s reputation and elevates his own status in the public eye. In a cunning scheme, Lisbeth, now incognito, retrieves two billion euros from Wennerström's hidden accounts, culminating in his murder during a gang shooting shortly after.
As the story concludes, Lisbeth witnesses Blomkvist, now entangled with his romantic interest Erika Berger. Overcome with a mix of emotions, she discards the Christmas gift she had intended for him and drives away on her motorbike, leaving a trail of unresolved feelings and shadows of her tumultuous journey behind.
