How do you keep a movie franchise from growing stale? One way is to put a different director on the case, bringing a fresh eye - and, ideally, a set of fresh ideas - to what might otherwise seem like yet another vacation to the same place you visited last year.
Accordingly, filmmaker Chris Columbus has stepped out of the picture after guiding the first two Harry Potter episodes to solid box-office success. More important, the director replacing him is Alfonso Cuarón, a versatile screen artist whose movies range from "Great Expectations" to the offbeat Mexican hit "Y tu mamá también."
Those pictures have a distinctively dark side, and advance word from Hollywood predicted that "Harry Potter and the Prisoner of Azkaban," based on the third of J.K. Rowling's novels, would be more sinister than the earlier films.
Is it ever. Stretching the PG rating to its breaking point, Mr. Cuarón has directed the creepiest and scariest Potter picture yet - and to my mind, the best.
Cuarón's originality does not show itself right away; at first, "Azkaban" is much like its predecessors. It opens with an obligatory scene of domestic dysfunction involving Harry's dislikable guardians, the Dursleys, and reaches strenuously for laughs with some mean-spirited magic that turns an obnoxious relative into a flyaway balloon. This is standard stuff, as are the early scenes at Hogwarts, the witchcraft-and-wizardry school Harry attends.
But evil doings are afoot, and after the first hour things grow steadily grimmer. A crazed killer has escaped from the Azkaban slammer, apparently for the purpose of murdering Harry himself.
Reporting more than this would reveal too many of the movie's secrets, so I'll only add that a werewolf, a magic map, and a black-robed executioner also play key roles in the story.
And, oh yes, a hippogriff - a mythical beast that has been brought to visual life with consummate skill, regenerating some of the specialness that special effects have lost in recent years. From its restlessly moving camera work to its heartfelt acting by a splendid cast, "Azkaban" is a horror movie for mature kids.
• Rated PG; contains scenes of violence and peril.