From http://en.wikipedia.org/wiki/The_Hobbit:
Gandalf tricks Bilbo into hosting a party for Thorin's band of dwarves, who sing of reclaiming the Lonely Mountain and its vast treasure from the dragon Smaug. When the music ends, Gandalf unveils a map showing a secret door into the Mountain and proposes that the dumbfounded Bilbo serve as the expedition's "burglar". The dwarves ridicule the idea, but Bilbo, indignant, joins despite himself.

The group travel into the wild, where Gandalf saves the company from trolls and leads them to Rivendell. While there, Elrond reveals more secrets from the map. Passing over the Misty Mountains, they are caught by goblins and driven deep underground. Although Gandalf rescues them, Bilbo gets separated from the others as they flee the goblin tunnels. Lost and disoriented, he stumbles across a mysterious ring and then encounters Gollum, who engages him in a game of riddles with deadly stakes. With the help of the ring, which confers invisibility, Bilbo escapes and rejoins the dwarves, raising his reputation with them. The goblins and Wargs give chase but the company are saved by eagles before resting in the house of Beorn.