For certain grown-ups in the audience, the most thrilling moment in ''Harry Potter and the Prisoner of Azkaban'' may well be the director's credit at the beginning. Alfonso Cuarón, who made the feverish and romantic ''Y Tu Mamá También,'' is a name sure to set many critics' hearts aflutter, even as the majority of hard-core Harry Potter fans, who tend not to see many unrated, subtitled pictures or to know the names of many foreign filmmakers, are likely to respond with a shrug.
The kids may also, at this point, be feeling a bit blasé about Harry himself. He will no doubt remain a beloved and profitable fixture of juvenile popular culture for years to come, but the mania that greeted the publication of the two latest Potter books (and the release of the first two movies) seems, at least for the moment, to have subsided. The first generation of Potterphiles has moved on to other forms of fantasy -- Philip Pullman's ''Dark Materials'' cycle, J. R. R. Tolkien's perennial ''Rings,'' the study manuals of Stanley Kaplan -- while their younger siblings now encounter the Potter series as a hand-me-down, rather than as their own special discovery.
All of which places Mr. Cuarón in an awkward position. As a filmmaker he is very much on the rise, while his hero may have reached a plateau. His accomplishment is thus in danger of being underappreciated. ''Harry Potter and the Prisoner of Azkaban'' -- which opens nationwide tomorrow, with some midnight showings tonight -- is the best of J. K. Rowling's books so far. It balances the narrative economy of the first two with the thematic depth and imaginative ambition of its ungainly sequels. And Mr. Cuarón's adaptation, from a screenplay by Steve Kloves, more than does it justice.
This is surely the most interesting of the three Potter movies, in part because it is the first one that actually looks and feels like a movie, rather than a staged reading with special effects. ''Sorcerer's Stone'' and ''Chamber of Secrets,'' both directed with literal-minded competence by Chris Columbus (who has stayed on as a producer) may have been more faithful to Ms. Rowling's text, but ''Azkaban'' attempts, and for the most part achieves, a trickier sort of translation. This film may disappoint some dogmatic Old Hogwartsians: a few plot points have been sacrificed, and Mr. Cuarón does not seem to care much for Quidditch. But it more than compensates for these lapses with its emotional force and visual panache.
Mr. Cuaron's wizard world, shot by the gifted New Zealand-born cinematographer Michael Seresin, is grainier and grimier than Mr. Columbus's. It feels at once more dangerous, more thoroughly enchanted and more real. While the two first episodes took place mostly in the corridors and classrooms of Hogwarts, this one lingers in the shadowy forests and damp meadows outside the school walls, a setting that emphasizes Mr. Cuarón's knack for evoking the haunting, sensual power of the natural world.
Harry, Ron and Hermione (played once again by Daniel Radcliffe, Rupert Grint and Emma Watson), hover on the brink of adolescence, and while they look braver and more capable than before, the dangers they face seem far more grave and their own vulnerability more intense.
At the beginning, in the obligatory Muggle-baiting scene, Harry's latent anger at his orphaned state (and at his cruel adoptive guardians) nearly overwhelms him. As the story leads him toward a confrontation with the man who may have killed his parents (an escaped prisoner played by the all-purpose movie villain Gary Oldman), Harry's happy-go-lucky decency is darkened by grief, rage and moral confusion.
Or at least they should be. Mr. Radcliffe, arriving at puberty, may also have reached the limit of his range as an actor. When called upon to convey deep or complex feelings, he has a tendency to blink and look nervous. To be fair, Harry is an especially treacherous role, since he is both the charismatic center of the drama and the character everyone watching imagines him- or herself to be. This means he has to be heroically distinguished from his peers without having too distinct a personality of his own, a paradoxical demand very few young actors could ever satisfy.
Luckily Mr. Radcliffe's blandness is offset by Ms. Watson's spiky impatience. Harry may show off his expanding wizardly skills in facing down dementors, werewolves and mad dogs, but Hermione, her fuse shortened by an impossible course load, earns the loudest applause with a decidedly unmagical punch to Draco Malfoy's deserving nose.
While the monstrous special effects are seamlessly inserted into the musty halls and twilight fields, ''Prisoner of Azkaban,'' like its predecessors, is anchored by top-of-the-line flesh-and-blood British acting. Michael Gambon, as the wise headmaster Albus Dumbledore, has gracefully stepped into Richard Harris's conical hat and flowing robes, and Maggie Smith shows up now and again to shake her head and purse her lips.
I am happy to report that this story has more Snape than the last, which means more chances to savor Alan Rickman's dry, sibilant sneering. New additions to the Hogwarts faculty include Emma Thompson as a daft, half-blind soothsayer and David Thewlis as an apparently kindly professor of spells with an uncomfortable secret.
''Harry Potter and the Prisoner of Azkaban'' is rated PG-13 (Parental Guidance suggested). It has some uncommonly sc
ary monsters and a dark, unsettling mood.
