Film 1 ends with the death of Smaug and a short, five minute aftermath in Laketown. At this point nothing much has happened with the white council, except that they have gathered, we have seen snippets of other threads (eg we see Aragorn as a youth) and they may have reached a decision about whether the necromancer is Sauron or not. We will also see Saruman as a good being, with a few subtle hints about his future.
Then, film two starts with the aftermath of Smaug's death from the dwarves' point of view. After ten or fifteen minutes of dwarven angst, we go back to the white council, who are getting ready to march off to Dol Guldur. They get involved in a few skirmishes on the edges of the forest, etc. Meanwhile, the whole Bilbo vs Thorin tensions rising plot thread is getting stretched to its limit, also we get a view from Bard's perspective of the Laketown dilemma. Back in Gandalf's plot thread, we follow the White Council as they fight their way to Dol Guldur, eventually we get to see the great battle between Galadriel and the Necromancer (I think that's right, I might be thinking of the sacking of Dol Guldur in the war of the ring) with Gandalf and Saruman and Elrond and the rest of the White Council pwning dark lord ass. There are a few cuts across to what's happening with Bilbo, then a twenty minute cool time after Dol Guldur is kaput, in which Gandalf wraps up and Bilbo hands over the arkenstone. The Battle of the Burning plains gets geared up, Gandalf arrives, there's some more pwnage, Thorin and his nephews bite the biscuit, and Bilbo mourns for a bit then goes home rich. Whether or not we see him actually return to the shire is irrelevant, really.
Anyway, what I can't figure out is how they will make the second installment take up three hours and yet keep the first one under three hours, when so much of the story is in part one. If it is. You never know, they might cut off the story just as they get to Smaug's lair, although starting a movie with Bilbo's encounter with him would undermine the significance and awesomeness of the moment.