This shorter and yet suspenseful novel written by Stieg Larsson and David Lagercrantz takes time over the course of five days to tie together stories that seemingly have no tie. The story is the fourth story in the Millennium series. This story follows Lisbeth Salander, a Swedish computer hacker, who accidentally discovers that her sister and father’s criminal organization is also working with a government agency, the NSA. Frans, a leading scientist in the AI and quantum computing fields, learns his only son, who is autistic, is being abused by his stepfather. Frans cannot stand by and let this happen so he takes a flight from America to Sweden to retrieve his son.

August, Fran's son, is malnourished and not properly cared for because his guardians have used his disability money on other things they wanted instead of things August needed. When Frans and August return to his home they find that August is actually a mathematical savant in art and mathematics. Frans does all he can to make sure that August continues to use his gift and encourages him to attempt harder math and more delicate drawings.

During the time in which Frans is retrieving August, Gabriella Crane discovers that Frans could be in danger and sends a security team to him to try and save his life. Gabriella learns of this because she is an analyst for the Swedish Security Police, SAPO. Frans refuses to leave to go to the safe house so Gabriella arranges for security patrols to run to keep Frans and August safe. Frans makes a few calls out including one to The Millennium, a local newspaper.

When the reporter arrives to the house to interview Frans about some secrets, the reporter finds that Frans is already dead. Frans was killed by a hitman hired by Spider to kill him. The murderer leaves August alive since August can’t speak. Mikael is a washed up journalist for The Millennium, who is going broke and may have to shut down. The company is then bought by a bigger corporation and is told that if they don’t gain new readers then the corporation will change the “essence” of The Millennium.

With this news heavy on Mikael’s heart he decides to follow up on the Frans murder, hoping that this story will be what keeps their doors from closing. The Swedish police investigate the murder of Frans further, even though there is a set back within their ranks due to secrets being leaked that place in jeopardy several operations. Salader hacks herself into the NSA mainframe and steals secrets about her sister and father’s organization, named, Spider. Along with those secrets Salander finds out that the NSA has ties with Spider. Not only ties with Spider but also with the company that Frans worked for before he died.

August, since his father's death, is returned to his abusive mother who puts him in a hospital for medically needy children. This is so she doesn’t have to care for him anymore but will still continue to receive her government checks. While August is in the medical facility they do not allow him to draw at all. Many people realize shortly, though, that August can technically help the officers by drawing a picture of the guys who murdered his father. Because of the NSA leaks, the Spiders have set out to kill August. Spider sends a fake email pretending to be the DR. When Salander realizes that this a trap designed to take August. Salander races to the institution and barely makes it there in time. She takes a shot to the shoulder when she rescues August from the hitman.

Erika is in the getaway car that August and Salander are in. Erika arranges for the two of them to go stay in a safe house, Erika’s summer home. While Salander and August are at the home Salander decides to give August some paper in case he is able to draw what happened that night. Salander even encourages him to solve complex mathematics again. Once August feels safe and secure, he sits down and draws a detailed sketch of his father’s killer.

Mikael develops a way to communicate safely with Salander and August with some help from Mikael's assistant, Zander. One night Mikael goes out for a drink and asks his assistant to go but she declines in favor of staying and working on the story. While Mikael is out he bumps into a beautiful woman who eventually seduces him and takes him to her apartment. Mikael doesn’t feel good about this so he leaves. Zander is then lured to the apartment and beaten and tortured by this woman’s colleagues.

The woman in charge of the kidnapping and interrogation is Salander's sister, the leader of the Spiders. They torture Zander to find out the whereabouts of August and Salander, which she eventually gives to Spider. The hitman ends up killing Zander. Spider uses this information to hunt down Salander and August at the summer home. August sees the hitmen outside his window and screams. This scream alerts Salander and she runs out and shoot the hitman, who turns out to be the same man that killed August’s father based on August’s drawings. Salander then sneaks around and kills two other men who are waiting to kill them, and then she escapes with August in tow.

After escaping the shooting at the summer home Salander takes August to his mother to be safe and runs off the stepfather who is abusive. He is 'persuaded' to never come back. Salander then uses the money to help pay for August and his mother to have a good life somewhere safe. Salander keeps in touch and August helps her crack the mathematical code on the NSA file she couldn’t figure out. This allows for Salander to expose the NSA. The story is covered by Mikael who single-handedly saved the company. When he returns from a party, Salander is waiting on his doorstep.