"The Girl with the Dragon Tattoo" is a Swedish film based on the book of the same name written by Stieg Larsson. The movie is the first in a series of three based on Larsson's Millennium trilogy. In the movie, Mikael Blomkvist is a journalist who has recently lost a court case regarding charges of libel against a multi-billionaire businessmen named Wennerstrom. Disgraced and broke, he accepts a job from the elderly patriarch of the Vanger family, Henrik Vanger. Henrik asks Blomkvist to investigate the disappearance of his favorite niece, Harriet, more than 35 years ago during a joint family reunion and company board meeting. He suggests that Blomkvist uses the cover of writing a biography about his life because he suspects that one of his relatives is responsible for the kidnapping and murder of young Harriet. Despite the use of the cover story, the Vanger relatives are aware of Blomkist's real intentions in interviewing them and he faces their hostility.
Blomkvist makes a breakthrough with the realization that Harriet's deceased father was potentially a serial killer and that she may have been killed by someone trying to cover up her father's crimes. He enlists the aid of Lisbeth Salander, the young woman that performed the background check on Blomkvist for Vanger. Blomkvist is aware that Salander must be a very skilled computer hacker because she was able to find out information that was kept only on his personal laptop. He appeals to her sense of justice to persuade her to work with him.
Alongside Blomkvist's investigation of the Vanger family, Salander struggles with her legal status as a ward of the state. When her elderly social worker is hospitalized after a stroke, she is assigned a new social worker who abuses his authority. This motivates her to become more independent.
Together she and Blomkvist unravel the mystery of Harriet's disappearance and her family's involvement in the string of murders.