Roy and Marth aren't in it (well you can get them as downloadable units, but they don't take part in the story)
Though it's the only one I have ever played my understanding of the Fire Emblem games are that they are each based on a different war over the course of whatever planet it is history. Different cast for the most part each time.
Great game though, real time sink and lots of customisation.
I've only had the 3DS since October, enjoying it quite a bit, plus it's been useful catching up on all the DS games i've missed.