IT’S HARD TO know what’s worse: having Dementors guard the wizards at Hogwarts School or not having them. Dementors stand ready to recapture Sirius Black, who escaped from Azkaban prison, but they take humans’ pain and force them to relive their worst memories. So although it’s a pleasure to be back at Hogwarts after another dreadful summer at the Dursleys’ house, Harry Potter, 13, must reckon with being protected from Sirius (Gary Oldman), who seems to want him dead.

Welcome to the world of Harry Potter, as imagined by novelist J.K. Rowling and as portrayed by the excellent young actor Daniel Radcliffe.

With Harry Potter and the Prisoner of Azkaban, the HP franchise makes a couple of shifts. Following Richard Harris’ death, the fine actor Michael Gambon assumes the mantle of the beloved headmaster, Albus Dumbledore, and wears it well. New characters include Professor Lupin (David Thewlis) and the Divination teacher, Sibyll Trelawney (Emma Thompson, wearing a comedia mask of soda-bottle glasses). John Williams’ music mercifully stays in its place.

The biggest change, however, is in direction: Chris Columbus, director of the first two films, took a producer’s chair, and Alfonso Cuarón rode his director’s chair from Mexico to England. Cuarón co-wrote and directed “Y Tu Mama También,” which admirably transcends from noisy sex to silent mourning.

His deftness is not quite so manifest in “Harry Potter 3.” Gone is the balance between the sweet and the scary. The silly bits – Dawn French wails as a fabulous fat lady, and Hermione (Emma Watson) manages to be in two classes at once – flit by, and there’s but one game of quidditch, played in fog.

It helps to have read the book; otherwise, Hermione’s comings and goings don’t make much sense, and Lupin’s role seems odd rather than a mystery in search of a solution. Cuarón guides the theme of searching for a father through Harry’s discoveries of Sirius’ identity, of Lupin’s role and of his own power. The theme culminates with Sirius’ assurance – and a very Christian one it is, too – that “the ones who love us never really leave us.”

The director respects Rowling’s emphasis on Hermione’s intelligence and compassion, while he delights in the trio’s interconnection. His understanding of adolescents reigns throughout: Hermione loses the tight curls from the first film of the series, the students at Hogwarts dress like little slobs, and Harry grins like mad as he soars above the castle.

Although Cuarón has a deft hand with lighting (the evergreens’ candling is lovely) and with scenery (beautiful mountains and vales and lakes), this film depends greatly on special effects. They are done well, especially the movements of the Hippogriff (bird + horse) and the shapeshifting of a rat into a man (Timothy Sprall) and a man (Sirius) into a dog.

Harry Potter and the Prison of Azkaban works well as the bridge between Harry’s preadolesence and the first signs of maturation. The film is not the book, but it’s a darn good film.