In J.R.R. Tolkien's The Hobbit, Bilbo Baggins's mission is to accompany Thorin Oakenshield and his company of 12 other Dwarves (13 total), led by the wizard Gandalf the Grey, on a quest to the Lonely Mountain (Erebor) in the East.
Gandalf chose Bilbo as the company's "burglar"—a role requiring stealth, cunning, and luck to infiltrate Smaug's lair. Bilbo's discovery of the One Ring in Gollum's cave and his riddle-game with Gollum, his rescue of the Dwarves from spiders and wood-elves, and his pivotal conversation with Smaug (finding the bare patch) fulfill this role.
The quest succeeds: Smaug is slain by Bard the Bowman; Erebor is reclaimed; the treasure is redistributed (after the Battle of Five Armies). Bilbo returns to the Shire changed—wealthier, wiser, and carrying the Ring.