What is this の doing at the end of the sentence?

I am reading a graded reader for the first time, and came across this sentence. I am curious what the の is doing grammatically at the end of the sentence before かな ?

この人たちは、 どこから来て、 どこへ行くのかな?

Thanks!

EDIT: Thank you for the explanations everyone.

by Thartperson

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