In “The Hobbit: An Unexpected Journey,”Peter Jackson’s adaptation of J. R. R. Tolkien’s first Middle-earth fantasy novel, Bilbo Baggins (Martin Freeman) sets out with the wizard Gandalf (Ian McKellen) and a posse of dwarfs to battle a fearsome dragon. [Spoiler alert] they do not kill the dragon, although [spoiler alert] they eventually will, within the next 18 months or so, because [spoiler alert] this “Hobbit,” which is [migraine alert] 170 minutes, is the first installment in [film critic suicide-watch alert] a trilogy.

What’s that old saying so memorably garbled by a recent president? Fool me twice — won’t get fooled again! This is not to say that Mr. Jackson is a con man. On the contrary: He is a visionary, an entrepreneur, a job creator in his native New Zealand. And his “Lord of the Rings” movies, the last of which opened nine years ago, remain a mighty modern gesamtkunstwerk, a grand Wagnerian blend of pop-culture mythology and digital magic now available for easy, endless viewing in your living room.

“The Lord of the Rings” was the work of a filmmaker perfectly in tune with his source material. Its too-muchness — the encyclopedic detail, the pseudoscholarly exposition, the soaring allegory, the punishing length — was as much a product of Tolkien’s literary sensibility as of Mr. Jackson’s commitment to cinematic maximalism. These were three films to rule them all, and they conjured an imaginary world of remarkable complexity and coherence. This voyage, which takes place 60 years before Frodo’s great quest, is not nearly as captivating.

Part of this has to do with tone. The “Rings” trilogy, much of which was written during World War II, is a dark, monumental epic of Good and Evil in conflict, whereas “The Hobbit,” first published in 1937 (and later revised), is a more lighthearted book, an adventure story whose comical and fairy-tale elements are very much in the foreground.

The comparative playfulness of the novel could have made this “Hobbit” movie a lot of fun, but over the years Mr. Jackson seems to have shed most of the exuberant, gleefully obnoxious whimsy that can be found in early films like “Meet the Feebles” and “Dead Alive.” A trace of his impish old spirit survives in some of the creature designs in “The Hobbit” — notably a gelatinous and gigantic Great Goblin and an encampment of cretinous, Three-Stooges-like trolls — but Tolkien’s inventive, episodic tale of a modest homebody on a dangerous journey has been turned into an overscale and plodding spectacle.

Also, not to be pedantic or anything, but “The Hobbit” is just one book, and its expansion into three movies feels arbitrary and mercenary. This installment takes Bilbo and his companions, led by the exiled dwarf king Thorin (Richard Armitage), son of Thrain, through a series of encounters with orcs, elves, trolls and other beings, some scarier or more charming than others. The only character who manages to be a bit of both is the incomparable Gollum, once again incarnated by Andy Serkis in what remains an unmatched feat of computer-assisted performance.

The meeting between Bilbo and Gollum, which takes place in a vast, watery subterranean cavern, is the one fully enchanted piece of “An Unexpected Journey.” It’s a funny, haunting and curiously touching moment that summons the audience to a state of quiet, eager attentiveness. Even if you aren’t aware of the apocalyptic importance of Gollum’s precious ring, you feel that a lot is at stake here: Bilbo’s life and integrity; Gollum’s corroded soul; the fate of Middle-earth itself.

If only some of that feeling animated the rest of the movie. There are, of course, plenty of shots of noble characters turning their eyes portentously toward the horizon, and much talk of honor, betrayal and the rightful sovereignty of dwarfs over their dragon-occupied mountain. But it all sounds remarkably hollow, perhaps because the post-“Lord of the Rings” decade has seen a flood of lavish and self-serious fantasy-movie franchises. We have heard so many weird proper names intoned in made-up tongues, witnessed so many embodiments of pure evil rise and fall and seen so many fine British actors in beards and flowing robes that we may be too jaded for “The Hobbit,” in spite of its noble pedigree.