The eighth installment in the Millennium series, originally created by Stieg Larsson and continued by David Lagercrantz and then Karin Smirnoff, picks up months after the previous novel,
The Girl in the Eagle's Talons. In that book, Lisbeth Salander's niece Svala Hirak carried her dying mother, Märta Hirak, out of a burning bunker in the northern Swedish town of Gasskas, where white supremacist crime lord Marcus Branco had held her captive.
In December, Lisbeth's longtime hacker ally Plague phones her, claiming to check in. Lisbeth senses something false in his tone. Plague has failed to hack the computer systems of the Branco Group, Branco's security company and criminal network, and considers himself a traitor, though he cannot confess. For Christmas, Lisbeth sends Svala a surveillance drone, a gift that will prove significant.
The following May, a terminally ill man detonates an explosive beneath an eighteenth-century bridge near the decommissioned Gasskas mine, known locally as "the Pit." Meanwhile, Svala, now interning at the local newspaper
Gaskassen, works with journalist Ester Södergran on a feature about abandoned properties. They visit a former sanatorium posted as private land. Svala uses Lisbeth's drone to photograph the building and discovers silhouettes of human figures through the skylights. Ester's hacker contact, Ante, discovers the sanatorium is owned by Mimer Mining, the parent company behind proposed new mining operations.
At an activist meeting opposing the mine, a newcomer named Simon Frisk, who claims to be studying energy engineering but is secretly an operative recruited by Branco to infiltrate the activists, advocates physical resistance. Svala is drawn to his charisma. Days later, Ester's body is found at the bottom of a crater at a decommissioned dump. Police officers Jessica Harnesk and Birna Guðmundurdottir discover the corpse, but the cause of death remains undetermined.
The novel introduces Lo, the code name Branco has given to Marika Vikström, a woman with extensive burn scarring from a childhood house fire who serves as his primary operative. Lo infiltrates the care home where her own father, Kurt Vikström, has been placed after he witnessed men drag a girl to the crater and found a body the next morning. She smothers Kurt with a pillow, eliminating the only witness. The wheelchair-bound Branco then reveals his obsession: Svala's late mother owed him a hard drive containing Bitcoin now worth billions of kronor, and he believes Svala can crack its passcode. He orders Lo to surveil Lisbeth by joining her martial arts club and eventually using her to reach Svala.
In Stockholm, Lisbeth and Mikael Blomkvist, her longtime investigative partner, reconnect at a café. Mikael presents a document reconstructing the previous autumn's events. Lisbeth tries to contact Plague and gets no reply. She visits his basement dwelling and finds it ransacked, his computer missing. At karate practice, Lisbeth spars with Lo, who introduces herself as Louise, claiming to have moved back from London with her family. Lo's burn-scarred body and fighting skills deepen Lisbeth's suspicion. Later, at a bar, Mikael discloses that he has prostate cancer and has been offered the editor-in-chief position at
Gaskassen.
Plague calls Lisbeth late one night, warning her that their shared platform has been hacked. He is then abducted and transported north to the sanatorium, where Branco's people sever one of his fingers, the one bearing a ring engraved with a pi symbol that Lisbeth gave him. The finger is mailed to Lisbeth in a matchbox. Mikael receives a threatening package containing an animal's eye. Both realize they are being targeted.
In Copenhagen, Joar Bark, known as the Cleaner, Branco's former contract killer and estranged brother of Gasskas municipal CEO Henry Salo, receives his final assignment: kill Douglas Ferm, a wealthy industrialist with mining connections who has been covertly feeding information about Branco's network to journalist Mikael Blomkvist. Ferm reveals he has investigated Branco's network and warns that his death would trigger the release of compromising information. The Cleaner spares Ferm and begins planning to turn against Branco.
Svala falls deeper under Simon's influence. He redirects a planned meeting to a hotel room in Jokkmokk, where he sexually assaults her. Svala steals his phone and flees. Consulting the imagined presence of her dead mother, whom she calls Mammamärta, Svala identifies Simon as a former associate of her dead stepfather's criminal network. She visits the office of Salo, who advocated reopening the mine, and presses him about her mother's death. Salo essentially confesses that his political decisions led to Märta's capture and murder, justifying the outcome as the cost of progress. Svala forms a wary alliance with the Cleaner at an abandoned cabin. He proposes using her as bait; she counters with her own demand: help settling scores for her mother's death.
Meanwhile, Kostas Long, a Greek-Chinese businessman, comes to dinner at the Salo home. Pernilla, Salo's wife, recognizes Long as her former abusive partner from Greece and the biological father of her son Lukas. Long demands custody of Lukas in exchange for financing the mine project. Salo promises to handle the situation but privately wavers.
Pernilla, alone at the family cottage, is attacked by an intruder and escapes on a snowmobile across breaking spring ice. Per-Henrik Hirak, Svala's uncle, rescues her and brings her to Björkavan, the Hirak family farm. That same night, Salo, consumed by guilt after revealing Pernilla's location to Long, walks onto the lake ice and drowns. Long arrives at the Salo home to claim Lukas and attacks Mikael. Lisbeth arrives and holds Long at gunpoint, but he escapes.
On Svala's fourteenth birthday, Lisbeth presents her with a captured Simon Frisk, bound in her hotel room. Svala holds a gun to his head but does not pull the trigger. She reveals Simon works for Branco as an infiltrator among the activists and gives Lisbeth a USB stick containing Ester's investigation files. Days later, analysts at Milton Security, Lisbeth's employer, discover that "Ante" is actually Simon using a false identity. Svala, unaware, goes to meet "Ante" at the mine. When Simon appears, she directs him along a forest road and detonates a pre-placed explosive given to her by Levi Grundström, a dying activist who entrusted her with the detonator before succumbing to illness. The blast kills Simon and destroys the mine road.
Lo contacts Svala and arranges for her to enter the sanatorium voluntarily, promising that surrendering the hard drive will protect her family. Inside, Branco hosts the Nordic Assembly, a gathering of wealthy white supremacists, and displays Svala onstage in traditional Sami clothing while an attendee measures her skull with a caliper, invoking Sweden's historical race-science practices. Svala spots Douglas Ferm among the audience. Plague, kept alive as a useful hacker, is brought to Svala. When Branco's enforcer Varg kicks open the door and fires, Plague throws his body over the girl. The shot kills him.
The Cleaner and Lisbeth enter the sanatorium through underground tunnels. Lo shoots Svala's uncle Elias Hirak, who has come with Per-Henrik to rescue her, then confronts Lisbeth. As Lo prepares to fire, the Cleaner shoots Lo dead. Lisbeth finds Plague's body in the final room and whispers goodbye, then feels Svala's fingers moving beneath him. Per-Henrik helps lever Plague's body off the girl. Svala is alive but badly injured.
At Elias's funeral, the survivors reckon with their losses. Per-Henrik will continue raising Svala. Mikael learns his cancer has not spread. Svala tells him that Ferm attended the Nordic Assembly, upending Mikael's trust in his source. Jessica and Birna bring Svala a rescue puppy she names Laika. Branco's wheelchair is found in the river, but no body is recovered.
In an epilogue set on Election Day, September 2022, Branco travels through Stockholm's airport under a false name and checks into a luxury suite in Quebec, where an underage girl waits on the bed. The novel closes with the words, "The cruel one has awoken" (358), signaling that Branco has survived and his predatory violence continues unchecked.