On the contemporary screen, science fiction and fantasy are the explanatory genre. What would “The Matrix” have been without tight shots of Laurence Fishburne’s face earnestly recapping how things work and what’s at stake every 10 minutes, other than a full reel shorter?

“Eragon,” an adaptation of the wildly popular first book by Christopher Paolini in a proposed trilogy (the third installment has yet to be published) of sword and sorcery novels, does its best to keep the facts, faces, and rules of its mythical world coming at an absorbable rate. Director Stefan Fangmeier, a visual effects supervisor here making his debut in charge, has the good sense to dovetail an exposition-heavy prologue directly into the film’s credits. It’s an old-fashioned genre movie trick that should be revived more often.

What we learn alongside the names of the cast and crew is that the mythical kingdom of Alagaesia has been violently purged of the Dragon Riders, a noble and elite crew of warrior-sorcerers who telepathically communicate with their rides. The villain is King Galbatorix (John Malkovich), a traitorous, power-hungry ex-Dragon Rider. It has been so long since dragons flew close air support for the vanquished forces of good that when a young freedom fighter liberates a dragon egg from Galbatorix’s castle, Eragon, the farm boy into whose hands fate delivers the egg, mistakes it for a stone.

When the credits end and the stone pops open to reveal a CGI baby dragon who acquires the name Saphira and the telepathic voice of Rachel Weisz, it becomes obvious to everyone except Eragon that he is about to initiate a Dragon Rider revival. Close on the heels of that revelation is the news that Brom (Jeremy Irons),a sort of adult dropout from the village down the road, is himself a fallen Dragon Rider. Before you can say “Joseph Campbell,” Eragon, Brom, Saphira, and Arya, the freedom fighter, are on a road of trials leading them to a rendezvous with anti-Galbatorix warriors in a secret mountain headquarters.

Mr. Fangmeier sets things moving by keeping the mostly bloodless fight scenes coming and growing in complexity. A climactic set piece involving fire-breathing air-to-air combat is particularly well imagined and realized. The cast, headed by newcomer Ed Speleers and sensibly stocked with take-the-ball-and-run-with-it pros like Mr. Irons and Robert Carlyle as a bad sorcerer named Durza, also do their best to move the story forward and make the blizzard of names, laws, and terminology that are manna to the six-sided dice set seem like they carry some meaning for the rest of us.

Mr. Speleers balances the kind of nonthreatening masculinity that Leonardo DiCaprio perfected in “Titanic” with a circumspection that fits his character somewhat better than his sword. As Eragon and Mr. Irons’s Brom quarrel, spar, and eventually bond in widescreen closeups, their relationship can be summed up as thin-lipped age versus full-lipped youth. Sienna Guillory is also clear-eyed and energetic, if a little gaunt for chain mail, as Arya.

The filmmakers make no secret of the fact that “Eragon” is the first of three books and therefore, they hope, the first of three movies. It would seem that King Galbatorix is to get more directly involved in the saga further down the road, as Mr. Malkovich’s screen time is comparatively small. But that doesn’t keep him from jamming as many tongue-flicking verbal ticks as possible into each of the few lines he has. If “Eragon” does spawn sequels, the producers may wish to consider product-placement opportunities like adorning Galbatroix’s robe with Boar’s Head or Dak logos.

As espoused by Campbell, George Lucas, George Miller, and screenwriting guru Christopher Vogler, heroes’ journeys of this type have predetermined narrative thresholds that all must be crossed or addressed in the correct order. These dozen or so steps with names like the Call to Adventure, the Ordeal, and the Inmost Cave have easily recognizable analogs in story after story and film after film. The step that is usually the toughest hurdle for an audience to cross is the one known as the Refusal of the Call.

When Max in “The Road Warrior” tells the post-apocalyptic desert pilgrims who would be led by him to go save themselves, it seems like a logical thing for a monosyllabic ex-cop who lives in a car to do. When Eragon, a red-blooded kid with nothing to keep him down on the farm but some hay bales, tells both his own personal mind-reading, flame-breathing, flying dragon and a gorgeous she-warrior to take a hike, it just seems ridiculous.

Worse, he keeps right on doing it. As “Eragon” wears on, our hero continues to make dumb choices and have murkily motivated changes of heart. In so doing, he threatens the credibility of characters that have assembled to celebrate his worthiness, and the patience of the audience who has assembled to root for him in his struggles. Even Luke Skywalker had the good sense eventually to stop whining, kiss his sister, and get with the derring-do program.

Since it was published in 2003, “Eragon”has spent nearly 90 straight weeks on the New York Times bestseller list. What has made it even more of a publishing phenomenon is that its first-time author, Mr. Paolini, was barely 17 when his book was published. With its PG-rated swordplay, unambitious character finishing, and closing-credits song by prefab punkette Avril Lavigne, it would seem that “Eragon” the movie was created to appeal to Mr. Paolini’s less imaginative or industrious peers, or possibly their younger siblings.