Although you may find this quite difficult to believe – I myself had a great deal of trouble wrapping my head around this development – at the opening of Super Mario Galaxy 2, our hero Mario is on his way to the castle to meet the Princess. Rather than getting to sit with her for a delicious afternoon of tea and cake (or whatever), Peach is kidnapped by Bowser.
Oh yes, you heard correctly – Super Mario Galaxy 2 starts off with Peach being kidnapped by Bowser. Mario is quickly off to space to save her, because that's where giant Bowser (did I not mention that he's giant because he's found special bonus powers in space?) took her.
In all seriousness though, Super Mario Galaxy 2 progresses in a very similar fashion to 2007's Galaxy, a game which added a fantastic, spherical, twist to Mario's universe. The goal, as indicated above, is tried and true Mario – beat Bowser and rescue Peach. Mario achieves this by taking a small ship (that looks like Mario's head) around space, visiting different galaxies and worlds, and capturing stars. No, there's not a great deal of depth to the story, but there doesn't need to be either (it's like Link needing to get the Triforce to beat Gannon). Mario saves Peach. That's what he does, and that's probably all anyone needs him to do – the question is what does Mario have to do in order to save Peach? What worlds does he go to? How do levels play out? Mario games are more about mechanics and level design than story, and in Super Mario Galaxy 2 those elements are truly fantastic.
Perhaps the biggest difference between this new Galaxy and the original is that a classic Mario map has been placed upon the basic structure. The hub area is gone, Mario's small ship can now only fly to open worlds proceeding along the map. For this reviewer's money that concept is not well delivered. Yes, it does streamline some things to have the map, but plotting Mario's journey along a map removes the 3-D-ness of the levels. To some extent this is dealt with by Mario having the ability to not just look at a single "world map," but a "super world map" as well. That addition only highlights one of the basic flaws in the concept of the map.