Repeat: SPOILERS. Read at your peril.

In the movie (and the trailer, for that matter), Blomkvist says to Lisbeth, “I want you to help me catch a killer of women” and she gives him that kind of ice-cracking-vulnerable, you-had-me-at-tell-your-girlfriend-to-leave, look.

How does he know at that point that it’s a killer of women, rather than just the person responsible for Harriet’s disappearance/death? Doesn’t Lisbeth put those clues together for him afterwards, with Harriet’s coded clues? Is he only saying that to get her to help? And if so, isn’t that just an outrageous coincidence that he happens to be right?

In the Swedish version, this line isn’t there - Lisbeth helps him stealthily, without him asking, and puts him onto the right path. But in Fincher’s version I understand why Blomkvist seeks Lisbeth out, but not how or why he uses this line on her, which seems to make all the difference. The rest of the movie can’t happen unless this exact line convinces her to help, right?

What did I miss?