Lisbeth Salander is serving time in Flodberga Prison. When a sadistic gang leader nicknamed Benito starts to torture Faria, a young Bangladeshi prisoner, Salander finds it impossible not to intervene. Now a target of Benito and her gang, Salander's time in prison is further complicated by news from her old guardian, who tells her she may have been a subject in a secret experiment known as the Registry. She enlists her friend Mikael Blomkvist, crusading editor of Millennium magazine, to help her look into it.

Once Salander is released, she devotes her time to uncovering the truth in the case that landed Faria in prison, and Blomkvist continues his search into Salander's background. But when the two cases start to dovetail, Salander and Blomkvist must join together to fight for justice and their lives.