This is a long one, so check out the full poem here.
We start the second part of the poem "Many a weary year" after the Grand-Pré was burned to the ground. The villagers, in exile, were scattered all over the place: from the lakes up North (we're guessing these are the Great Lakes) to the Mississippi delta (Longfellow calls the Mississippi River "the Father of Waters) (675).
The now ex-villagers are a sad and miserable lot, searching for a place to call home (or, in some cases a nice, quiet grave to jump into). Among these, our speaker tells us, is a young, beautiful, but terribly sad girl. (Any takers on who this might be?)
This girl wanders from place to place, never staying too long in any one location. It seems that she's looking for someone. One of her pastimes is visiting unmarked graves, wondering if "perhaps in its bosom / He was already at rest" (696-697). If it is "him," then she just wants to die right beside him. Cheery, eh?
On occasion, rumors about this yet-unnamed fellow (again, place your bets if you have a guess) motivate her to search in one area or another.
Aha—we learn that it's Gabriel who this girl is searching for, so that makes this yet-unnamed girl Evangeline (shocker, we know).
Folks tell Evangeline to move on with her life; there are plenty of fish in the sea, etc., etc. Evangeline, though, is not having it. She wants Gabriel or nobody.
The priest (who apparently is still with her) encourages Evangeline to be true and patient. He tells her that all this waiting for Gabriel will make her heart more "godlike," and "more worthy of heaven" (726-727).
Evangeline feels encouraged by the priest, so she keeps pining away for the lost Gabriel.
Now our speaker pipes up. He asks the Muse (the divine source of poetic inspiration) to help him try to follow Evangeline's life. Using a simile, he compares his work with someone following a stream through a valley.
Sometimes the stream is near; sometimes it's far away, but it's always within earshot.
In other words: get ready for a jump ahead in our timeline…