Bilbo Baggins lives in a beautiful hole under The Hill, where he's a respected (and rather rich) pillar of the hobbit community. Hobbits, by the way, are small people with furry toes and big bellies. They like beer and jokes and generally making merry. They believe in hanging out and not making too much trouble. But even though Bilbo has spent his whole life living contently under The Hill, an adventure comes to his doorstep and takes him far away anyway.
It just so happens that Bilbo is sitting on his front stoop when Gandalf, a wizard, walks by. Bilbo invites Gandalf to come back for tea on Wednesday and then completely forgets the whole thing. On Wednesday at four, he hears the doorbell ring and thinks it's Gandalf – but it's actually thirteen dwarves: Thorin, Dori, Nori, Ori, Bifur, Bofur, Bombur, Oin, Gloin, Balin, Dwalin, Fili, and Kili. On Gandalf's recommendation, the chief dwarf, Thorin, has a job offer for Bilbo.
Thorin is the grandson of Thror, the dwarf who was King under the Lonely Mountain. Many years before, the evil dragon Smaug came and killed many of Thror's people and stole his treasure. Thorin wants to steal the treasure back from Smaug, and he wants Bilbo (who's evidently an expert burglar) to help him do it. Now, obviously, Bilbo is not an expert burglar. But he is so proud that Gandalf thinks so highly of him that he winds up agreeing to join Thorin & Co.
The thirteen dwarves, plus Bilbo and Gandalf, travel through the wilds to Rivendell, where they meet up with Elrond, a half-elf lord. Elrond shows Thorin & Co. some special hidden letters on Thorin's map of the Lonely Mountain. The letters (which are only visible at the right phase of the moon) say that the tiny, secret side door into the Lonely Mountain will only open on Durin's Day, the last day of autumn. So the dwarves decide to get a move on so that they can reach the Lonely Mountain in time.
On their way east, they must cross the Misty Mountain range. There, a bunch of goblins take them captive. Gandalf kills the Great Goblin and rescues all the dwarves. However, Bilbo gets separated from the group. As he makes his way as quietly as he can through the goblin halls, he happens to find a small golden ring in the dark. He puts the ring in his pocket and forgets about it.
Bilbo walks right into an underground lake. On an island in the middle of the lake is a small, slimy creature named Gollum. Gollum has been living under the mountain and dodging goblins for a long, long time. The one thing that makes his life bearable is his "birthday present" (5.78) – a golden ring that makes him invisible. And yes, that's the very same ring Bilbo has just found in the dark.
Gollum isn't hungry (yet), but he's not about to let Bilbo go past without a murmur. So Gollum challenges Bilbo to a game of riddles. If Bilbo wins, Gollum will show him the way out of the goblins' kingdom. If Gollum wins, Gollum gets to eat Bilbo. (No thank you.) Bilbo manages to win (barely), but Gollum still tries to kill him anyway. Luckily, Bilbo slips on the ring in his pocket by accident and becomes invisible. Gollum assumes that Bilbo is running out of the tunnels to the exit, so he quickly makes his way through the goblin kingdom. Bilbo follows Gollum with his ring of invisibility on. When they reach the exit, Bilbo leaps over Gollum and makes a break for the exit. He comes out on the other side of the Misty Mountains. Victory!
Bilbo hears voices nearby and rejoins his friends. Gandalf, Bilbo, and the dwarves are almost killed once more by vengeful goblins coming out of the mountains, but they escape just in time. Bilbo, Gandalf, and the dwarves seek shelter with a giant man, Beorn, who can take the shape of a bear. Beorn hates goblins, so he welcomes any goblin enemies to his home. Beorn gives Bilbo and the dwarves provisions to take them through the next stage of their journey: the giant forest of Mirkwood.
Gandalf leaves Bilbo and the dwarves behind as they enter Mirkwood. He has business elsewhere. But Gandalf warns them under no circumstances to leave the path and go into the forest. Of course, as they run out of food, they wind up doing just that. As they stray from the path, they stumble onto the feasts of the Wood-elves and Thorin gets taken prisoner while the rest of the dwarves scatter into the forest.
The rest of the dwarves (and Bilbo) get attacked by giant spiders. With his ring of invisibility and a knife, Bilbo manages to free himself and the dwarves. But the dwarves are then taken prisoner by the Wood-elves. So Bilbo quickly accompanies them to the Elvenking's dungeon, again while wearing his invisibility ring. As the dwarves sit in their thirteen separate cells, Bilbo manages to spy a way to escape. One night when all the elves are busy with a large feast, he frees the dwarves and packs them into barrels being taken to Lake-town. The dwarves arrive in secret in Lake-town, the nearest city to the Lonely Mountain.
Bilbo and the dwarves get assistance from the Master of Lake-town to travel to the Lonely Mountain. They settle down in front of the side door marked on Thorin's map. At last, Durin's Day comes and the door suddenly reveals a keyhole. Thorin uses his key to unlock the side door and open a tunnel into the heart of the Lonely Mountain. Bilbo uses this passage to creep down and steal a golden cup from Smaug the dragon. Bilbo also has a bit of a chat with the dragon (who can't see him, thanks to the handy ring), which is unnerving. Bilbo catches a glimpse of a bare patch of scales in Smaug's chest, which he realizes is the dragon's weak point. Bilbo tells the dwarves about this weak spot; luckily, a bird that will come into the story later hears this news as well.
Smaug, finding this one golden cup missing from his hoard of treasure, flies out to Lake-town in a rage. He wants to remind them all that he's the true King under the Mountain. But there is an archer in Lake-town, a descendant of lords of the town of Dale, which Smaug destroyed. The bird that has been keeping an eye on Bilbo and the dwarves flies over to Lake-town and finds this archer, Bard. It's the special skill of Bard's family to be able to understand this kind of bird. So when the bird tells him about the weak spot on Smaug's chest that Bilbo has discovered, Bard takes his great bow and his strongest arrow and waits until the dragon is flying right over Lake-town. He takes aim and shoots Smaug in the chest at just the right spot, and Smaug is killed.
Meanwhile, back under the mountain, Bilbo and the dwarves wait a while to see if Smaug is going to reappear. When he doesn't, Bilbo creeps down to his treasure. One of the first things Bilbo finds is a giant diamond, the Arkenstone of Thrain. This is a family heirloom that Thorin desperately wants. But Bilbo keeps it a secret from Thorin – he doesn't know exactly why, but he'll go with the hunch.
Tidings are spreading all over the place that Smaug has been killed. The Elvenking of Mirkwood comes with his armies in the hopes of grabbing some treasure; Bard also leads an army of men from Lake-town seeking money to rebuild their town and his own city of Dale. But, even though Bard is the one who killed Smaug and the Elvenking is Bard's friend, Thorin absolutely refuses to give them any share of the treasure. Thorin sends for his cousin Dain and some dwarvish troops to stand by his side. So it looks like they're going to come to war over Smaug's old treasure.
Bilbo wants to prevent this war and save his friends, so by cover of night, he brings the Arkenstone down to the camp of the humans and the elves. He gives it to Bard so that Bard can use it to negotiate with Thorin. But before negotiations between the humans, elves, and dwarves can continue, Gandalf suddenly appears with terrible news. While they have all been caught up with each other, the goblins of the Misty Mountains have been banding with the evil wolves (the Wargs) of the wild to march against all of them.
So, suddenly, the humans, elves, and dwarves who had been about to go to war against one another band together against the goblins and the Wargs. They barely manage to win, but they come out on top thanks to the help of a group of giant eagles and the sudden appearance of Beorn the bear-man. Thorin is horribly wounded in battle, and his last wish is to befriend Bilbo once more. He realizes that Bilbo was only trying to help, and after all, what is gold truly worth when you can't take it with you? Bilbo weeps at Thorin's death, and feels just about ready to go home. So Gandalf escorts Bilbo back to his old home under The Hill with a portion of the treasure and loads of respect from men, elves, and dwarves for his courage and resourcefulness.