The following morning, Blomkvist and Salander consult about their next steps in the investigation. Blomkvist states that he thinks Cecilia is telling the truth about not being in Harriet's room. Together, he and Salander theorize that there may be two people involved in the crime. Once again, they review the photographs Vanger has provided them for clues. They also review the biblical connections in an effort to connect Tjorven's slaughter with one of the passages regarding animal sacrifice. During their discussion, Blomkvist realizes that Salander has a photographic memory. His surprise about her abilities angers Salander, who rushes off to think by herself.
After Blomkvist and Salander make up, they walk to the church to look for more clues and discover the Vanger family crypt. Once inside, they find the site of Tjorven's dismemberment. They make a plan for the day: Salander will head to the Vanger Corporation archives to review old news releases to see if she can make a connection between the murders and the company. Meanwhile, Blomkvist will see if he can look at more of Vanger's photographs from the day Harriet disappeared.
Blomkvist is able to find two important clues by looking through Vanger's photo albums: Anita Vanger was also present on the island the day Harriet disappeared, and Anita and Cecilia both looked alike and dressed similarly that day. These observations, along with the fact that Anita was known to be a close friend to Harriet, suggest that Anita was the one who opened Harriet's window. Blomkvist also finds a picture of a more youthful Martin that makes him realize that it was Martin who Harriet saw the day of the parade.
Salander also makes headway in her review of old Vanger Corporation materials and discovers a Vanger company near all the murder victims' locations. From there, she is able to trace Gottfried to many of these same locations around the time of the murders. Salander is still puzzled about the murders that occurred after his death.
Back at the cabin, Blomkvist is restless. It is early evening and Salander is not home yet. Blomkvist desperately wants to confront Martin, but realizes the folly of the idea if Martin is the killer. Nonetheless, he heads over to Martin's house, and Martin quickly takes control of the situation. He leads Blomkvist into a secret, soundproof chamber in the basement of his house. He handcuffs Blomkvist, beats him up, and they then discuss the history of Martin's murders. Martin confesses he has every intention of killing Blomkvist and, once Salander returns to the cabin, her as well.
Tension mounts in Chapter 23, and the theme of trust is explored through Salander and Blomkvist's trust in one another's instincts. First, Blomkvist's belief that Cecilia is telling the truth reinstates his shaky trust in her. Salander and Blomkvist open their minds to new alternatives, which help them see their evidence in a different light. The researchers arrive at a new theory: There may be two people involved in Harriet's disappearance. Secondly, although Salander is taken aback by Blomkvist's commentary on her photographic memory, she's able to recover and admit she trusts him, which is what allows the two to continue their partnership.
Much of Martin's true nature begins to be revealed in this chapter. Martin proves to be a Dr. Jekyll and Mr. Hyde type character: a calm, quiet CEO by day and a vicious, methodical, and ruthless killer by night. These two sides become apparent in Blomkvist and Martin's dialogue once Martin has handcuffed Blomkvist to the floor. Martin oscillates between calm and violence, and Blomkvist sees how very much he underestimated Martin's sociopathic nature. Additionally, Blomkvist's decision to visit Martin on his own contrasts with Salander's preparedness. Blomkvist, having never faced the difficulties Salander has experiences, is not as well-tuned to the necessity to think out consequences for his actions. His decision to go unarmed and unprepared to Martin's house is in contrast to Salander's decision earlier in the novel to face Bjurman — who is very much similar to Martin — at his apartment. While she, too, underestimated her foe, Salander at least came prepared with a video camera.