Well, we can speculate. Bilbo and the Necromancer have a furious swordfight in Dol Guldur and when Bilbo is nearly defeated the Necromancer reveals himself and sez, "Bilbo, I am your father." Bilbo escapes, steals the golden idol head, and runs like mad from Dol Guldur pursued by a giant rolling boulder. Then he throws a bomb down a ventilation shaft and the entire evil castle blows up. Alderaan is saved.
Wait, scratch that. The Shire is saved. I'm still working on the screenplay.