An interesting part of J.K Rowling's genius is that her children, Harry, Ron and Hermione, come to face more and more grownup challenges in each succeeding book. They are older, life at Hogwarts holds few surprises for them or us now, and so she's written more adult challenges for them. In their third year at school their world faces outward now, as much as it used to face in toward the daily classes, the bullies, the qwidditch match. Fears that were just hinted at in the earlier books are now visible to us and to Harry and must be confronted. The trio are now thirteen, have reached puberty (which brings its own problems), and Harry, particularly, is moving toward life (and death) issues that are spawned outside of Hogwarts.

"Harry Potter and the Prisoner of Azkaban" is a dark book, and screenwriter Steve Kloves and new (to the series) director Alfonso Cuarón - whose sexy "Y Tu Mama Tambien" rocked the film world a couple of years ago - has darkened the visual palette considerably. Sirius Black, a man who has been linked to the murder of Harry's parents, has escaped from Azkaban prison and may be a threat to Harry. The Dementors, frightful prison guards who suck the soul from their captives, are out looking for him - but may be looking for other people as well.

There are new teachers at Hogwarts. Professor Lupin (David Thewlis) has a secret that anyone who knows Rowling's propensity for root words should be able to guess. Professor Trelawney (Emma Thompson) can foretell the future. The deliciously snide Professor Snape (Alan Rickman) is back, as are Hagrid (Robbie Coltrane) and Professor McGonagall (Maggie Smith). A shaggy and bearded Michael Gambon takes over for Richard Harris as Albus Dumbledore with no loss of continuity. Hard-hitting Hermione gives the slimy bully Draco Malfoy a well-deserved punch in the face. Hagrid has a new pet this year, the Hippogriff, who is half horse and half bird. He, or it, will play an important role in the film.

But the plot has turned more sinister. There are revelations about Sirius, frightful encounters with the Dementors, and an ominous sense that Voldemort is coming closer. Cuarón has stepped up the pace of the film; he spends little time on life at Hogwarts and much more on shooting the action of the plot - so much so that we wonder whether Harry can do anything but stave off disaster. Most of it is set and staged at night and shot through a blue filter for a more ghost-like effect, but that is almost too much of a good thing; we wish for a little daylight, if only for a breather. And has Cuarón forgotten Hitchcock's lesson that the scariest things happen in bright sunshine and not in the dark?