Here's what happens in The Hobbit: The Desolation of Smaug. Don't worry: no spoilers in this part, just stuff from the book.
Thorin and Company visit Beorn, and ride borrowed steeds to the eaves of Mirkwood, where Gandalf leaves them to deal with Dol Guldur. In the forest, Bilbo climbs a tree to have a lookout. They're attacked by spiders, and the dwarves are captured by the Elves. Bilbo swipes a key and releases them, and they escape in barrels, and go to Laketown. At the Mountain, Thorin opens the side door with the key; Bilbo goes down and talks with Smaug; the dragon flies off to attack Laketown.
That's it. Nearly three hours long, and that single paragraph of plot is, as far as I can make it, the entire sum total of what this movie took from its book. Everything else in it, nearly three hours of it, is stuff PJ made up. Or changed beyond recognition; for instance, the two words "visit Beorn"? That's about all that that scene has in common with the book. A few things I left out of the summary may seem to be from the book, but they're not; for instance, the Elvenking interrogates Thorin, but the conversation is entirely different; and Bard appears, but he's also totally different (and, it turns out, a really bad actor).
And don't tell me it's all from LOTR, or the Appendices, or the Silmarillion. If I haven't missed anything, here's everything the movie got from there:
1) Flashback to Thorin meeting Gandalf in Bree;
2) Identification of the Necromancer with Sauron, and the consequent general threat to the peace of Middle-earth;
3) Legolas (who looks far older than he did in the LOTR films: do Elves age backwards?)
4) Brief allusions to Galadriel;
5) Athelas as a healing weed.
Here's just some of the stuff PJ made up, that's not from Tolkien at all:
(OK, now you can have some spoilers)
1) a band of orcs chasing the company to Beorn's, down the River Running, and all the way to Laketown, the better to transform the movie into an endless series of chase scenes;
2) the She-Elf (as the orcs call her), and her flirtation with Kili while Legolas stands around smoldering, looking more like her angry father than a spurned lover;
3) the dwarves slamming Beorn's front door shut in his own face; surprisingly, he's still willing to help them after that;
4) turning the Arkenstone into a McGuffin that confers the right to rule, and, incidentally, Thorin sending Bilbo down to steal that specifically, and since he doesn't manage it (partly because the dragon is there the first time), he accomplishes nothing at the Mountain as a burglar;
5) the dragon becomes Bard's fault because his ancestor, Girion, was a bad shot and failed to kill it (!);
6) the space between the last two items in my summary (Bilbo meeting Smaug and the dragon flying off) largely filled with an enormously long, utterly tiresome, and implausibly restricted (because PJ can't kill off any of the good guys at this point, much as they deserve it) series of scenes of the dwarves and the dragon chasing each other around inside the mountain. This turns out to be utterly pointless, as well, because it emerges that the dwarves' plan is to get their smelter turned on so that they can melt a huge pile of gold and pour it over the dragon, a la Auric Goldfinger. However, this fails either to kill the dragon or do anything other than piss him off, which becomes why, since he's contractually prevented from killing the dwarves, he flies off to burn Laketown instead;
7) an injured Kili, with a couple other dwarves to keep him company, left behind in Laketown (where they have no business being), attacked by orcs who have somehow managed to sneak onto Laketown (where they have no business being), and ridiculously surprise-rescued by Legolas and She-Elf (who also have no business being there).
Bilbo is a minor supporting character in this movie, and never gets a chance to be seen screwing up his courage and acquiring the maturity that the story is actually supposed to be about.
Gandalf is even more useless.
All the villains, whether played by Benedict Cumberbatch or not, speak in the same processed deep echoing villain voice that Jackson uses for just about all his villains.
She-Elf is your generic, right-out-of-the-box Warrior Princess, so if you like those, you'll like her.
None of the acting is very good - even Martin Freeman mostly just looks cross, and, for the first time in his entire career, Stephen Fry (as the Master of Laketown) is just ghastly - but for stiff, bad acting, the prize goes to the piece of wood playing Bard.
I am relieved to see that I felt far less bludgeoned over the head than at the previous movie. This is possibly because my eyes were rolling so far back in my head that I felt catatonic. But I couldn't have been, because I kept checking my watch in hopes that maybe the bloody thing would soon be over.
Look, I really don't mind PJ making up a story. It's your run-of-the-mill fantasy adventure story. I have just one complaint.
What is Tolkien's name doing on it?