The Hobbit: An Unexpected Journey is the first in The Hobbit film series
Bilbo Baggins, a hobbit of fifty years old, goes on an adventure with Thorin Oakenshield and his band of thirteen dwarves after being recruited by Gandalf the Grey.
Approaching his 111th birthday, the hobbit Bilbo Baggins begins writing down the full story of his adventure 60 years earlier for the benefit of his nephew Frodo. Long before Bilbo's involvement, the Dwarf king Thrór brings an era of prosperity for his kin under the Lonely Mountain, far to the East, until the arrival of the dragon Smaug. Destroying the nearby town of Dale, Smaug drives the Dwarves out of their mountain and takes their hoard of gold. Thrór's grandson Thorin sees King Thranduil and his Wood-elves on a nearby hillside, and is dismayed when they take their leave rather than aid his people, resulting in Thorin's everlasting hatred of Elves.
In the Shire, 50-year-old Bilbo is tricked by the wizard Gandalf the Grey into hosting a party for Thorin and his company of dwarves: Balin, Dwalin, Fíli, Kíli, Dori, Nori, Ori, Óin, Glóin, Bifur, Bofur, and Bombur. Gandalf's aim is to recruit Bilbo as the company's "burglar" to aid them in their quest to enter the Lonely Mountain. Bilbo is unwilling to accept at first but has a change of heart after the company leaves without him the next day. Travelling onward, the company is captured by three trolls, Tom, Bert & William, after their ponies are captured. Bilbo stalls the trolls from eating them until dawn. Gandalf exposes the trolls to sunlight, turning them to stone. They search the trolls' cave and find treasure and Elven blades. Thorin and Gandalf each take an Elf-made blade—Orcrist and Glamdring, respectively. Gandalf also finds an elven shortsword ("Sting"), which he gives to Bilbo.
The wizard Radagast the Brown finds Gandalf and the company, and recounts an encounter at Dol Guldur with the Necromancer, a sorcerer who has been corrupting Greenwood with dark magic. Gandalf leads the company through a stone passage to Rivendell. There, Lord Elrond discloses a hidden indication of a secret door on the company's map of the Lonely Mountain, which will be visible only on Durin's Day. Gandalf later approaches the White Council — consisting of Elrond, Galadriel and Saruman the White — and presents a Morgul blade Radagast obtained from Dol Guldur as a sign that the Necromancer is linked to the Witch-king of Angmar. When Saruman presses concern to the more present matter of the dwarves and Smaug, requesting that Gandalf put an end to the quest, Gandalf secretly reveals to Galadriel he had anticipated this and had the dwarves move forward without him.
The company journeys into the Misty Mountains where they find themselves amid a colossal battle between stone giants. They take refuge in a cave and are captured by Goblins, who take them to their leader, the Great Goblin in the heart of the mountain. Bilbo becomes separated from the dwarves and falls into a cave where he encounters Gollum, who unknowingly drops a golden ring. Pocketing the ring, Bilbo finds himself confronted by Gollum. They play a riddle game, wagering that Bilbo will be shown the way out if he wins or eaten by Gollum if he loses. Bilbo eventually wins by asking Gollum what he has in his pocket. Noticing his ring is lost, Gollum realizes that Bilbo possesses it and chases him. Bilbo discovers that the ring grants him invisibility, but when he has a chance to kill Gollum, Bilbo spares his life and escapes while Gollum shouts his hatred towards the hobbit Baggins.
Meanwhile, the Great Goblin reveals to the dwarves that Azog, an Orc war-chief who beheaded Thrór and lost his forearm to Thorin in battle outside the Dwarven kingdom of Moria, has placed a bounty on Thorin's head. Gandalf arrives and leads the dwarves in an escape and kills the Great Goblin. Bilbo exits the mountain and rejoins the company, keeping secret his newly obtained ring. The company is ambushed by Azog and his hunting party, and takes refuge in trees. Thorin charges at Azog, but is overpowered and left defenseless on the ground. Bilbo saves Thorin from the orcs just as the company is rescued by eagles. They escape to the safety of the Carrock where Gandalf is able to revive Thorin, who renounces his previous disdain for Bilbo after being saved by him.
- Martin Freeman as Young Bilbo Baggins: a hobbit hired by the wizard Gandalf to accompany 13 dwarves on a quest to reclaim the Lonely Mountain from the dragon Smaug.
- Ian Holm, who portrayed Old Bilbo Baggins in The Lord of the Rings trilogy appears also in scenes that take place directly before the events of The Fellowship of the Ring.
- Ian McKellen as Gandalf the Grey: a wizard who recruits Bilbo and helps to arrange the quest to reclaim the dwarves' lost treasure in Erebor. Gandalf was also portrayed by McKellen in The Lord of the Rings film trilogy.
- Richard Armitage as Thorin Oakenshield II: the leader of the Company of dwarveswho has set out to reclaim his birthright as King of the Lonely Mountain from Smaug.
- Ken Stott as Balin: Dwalin's brother. He is described in the novel as "always their look-out man".
- Graham McTavish as Dwalin: Balin's brother.
- Aidan Turner as Kíli: Thorin's nephew and Fíli's younger brother.
- Dean O'Gorman as Fíli: Thorin's nephew and Kíli's older brother.
- Mark Hadlow as Dori: Nori and Ori's brother. He is described in the novel as "a decent fellow, despite his grumbling", while Thorin described him as being the strongest member of the Company. Hadlow also plays Bert the Stone-troll.
- Jed Brophy as Nori: Dori and Ori's brother.
- Adam Brown as Ori: Dori and Nori's brother.
- John Callen as Óin: Gloin's brother.
- Peter Hambleton as Glóin: Óin's brother. Hambleton also plays William the Stone-troll.
- William Kircher as Bifur: Bofur and Bombur's cousin. Kircher also plays Tom the Stone-troll.
- James Nesbitt as Bofur: Bombur's brother and Bifur's cousin, described as "a disarmingly forthright, funny and occasionally brave Dwarf".
- Stephen Hunter as Bombur: Bofur's brother and Bifur's cousin; described in the novel as fat and clumsy.
- Cate Blanchett as Galadriel: the elven co-ruler of Lothlórien along with her husband, Lord Celeborn. She was also portrayed by Blanchett in The Lord of the Rings film trilogy.
- Hugo Weaving as Elrond: the Elven-Lord of Rivendell, who gives shelter to Bilbo's party, despite Thorin's great suspicion of Elves. He was also portrayed by Weaving in The Lord of the Rings film trilogy.
- Christopher Lee as Saruman the White: the head of the Istari Order and its White Council. He was also portrayed by Lee in The Lord of the Rings film trilogy.
- Sylvester McCoy as Radagast the Brown: an Istari wizard whose wisdom involves nature and wildlife.
- Elijah Wood as Frodo Baggins: Bilbo's favorite nephew. His scenes take place shortly before the events of The Fellowship of the Ring.
- Andy Serkis as Gollum: a wretched hobbit-like creature corrupted by the One Ring. Serkis portrayed Gollum through motion capture, as he did in The Lord of the Rings film trilogy. Serkis also acted as second unit director of the trilogy.
- Manu Bennett as Azog the Defiler: the Orc chieftain of Moria who beheaded King Thrór in the battle of Azanulbizar and now hunts for Thorin and his Company after taking an oath to break the line of Durin. He leads a band of Hunter Orcs and rides a huge white warg. He is shown as a large pale orc who, having had his left arm cut off, now wears a metal prosthetic hand and forearm.
- Lee Pace as Thranduil: the Elven-King of Mirkwood.
- Barry Humphries as the Great Goblin: the king of the underground caverns of Goblin Town in the Misty Mountains.
- Conan Stevens as a Gundabad Orc. He appears during the Battle of Azanulbizarfighting Dwalin.
- John Rawls as Yazneg: Azog's second-in-command. Movement choreographer Terry Notary played Yazneg during pick-up shots.
- Bret McKenzie as Lindir: an elf of Rivendell.
- Kiran Shah as the Goblin scribe: a scribe and messenger for the Great Goblin.
- Jeffrey Thomas as Thrór: the former king of Durin's Folk and Thorin's grandfather.
- Stephen Ure as Fimbul, one of Azog's Orc hunters, and lieutenant to Yazneg. After Yazneg is killed, Fimbul becomes Azog's right-hand man. Ure also played a goblin, named Grinnah, who acted as the Great Goblin's acolyte.
- Michael Mizrahi as Thráin II: the last Dwarf-King of Erebor and Thorin's father.
- Benedict Cumberbatch as the voice of the dragon Smaug, as well as The Necromancer, a mysterious sorcerer residing in Dol Guldur with the ability to summon the spirits of the dead. Cumberbatch provided performance capture for the character's brief appearance in this film.
Cameos in the film include director Peter Jackson and editor Jabez Olssen as Erebor Dwarf refugees running from the dragon Smaug in the opening prologue; picture double Hayden J. Weal as a dwarf carrying gems during Thranduil's visit in Erebor; James Nesbitt's then-wife Sonia Forbes-Adam as Belladonna Took, Bilbo's mother; Nesbitt's daughters Peggy and Mary as children of Dale; Luke Evans as Girion; and production designer Dan Hennah as the Old Took, Belladonna's father. Peter Jackson's daughter, Philippa Boyens's second son, and Andy Serkis' children made cameos in the Old Took's party; and Jabez Olssen's wife and children, Weta Workshop founder Richard Taylor's children, and set decorator Ra Vincent's children all cameo in the market scene. Writer for Ain'tItCoolNews.com, Eric Vespe, portrays Fredegar Chubb, the fish seller at the market. The cameos in the Old Took's party and the market scene are shown only in the extended edition.