The main plot is linear and you approach each of the four areas in a given order. However, there are a lot of sidequests that are optional and when you progress on the main story quests is up to you (hint: to max your cash, visit Venus between each main story quest). So I think it gives you a decent illusion of freedom. Unlike, say, Bioshock or even Deus Ex, where you are pretty much on the rails.
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