Harry Potter and the Prisoner of Azkaban marks a transition on numerous levels. The protagonists are in their last year of Hogwarts "middle school," so to speak, poised to make their move forward into the true throes of adolescence. The overall tone of the series shifts to an even darker place with the addition of the spectral Dementors. An aura of things hidden emerges with the Marauder's Map and its revelations, like unveiled clues in a mystery novel. And from the classroom exercise with the Boggart-occupied wardrobe, to Harry's efforts to successfully conjure a Patronus, to the transformations of Remus Lupin and the untruths surrounding Sirius Black, nothing can quite be counted on to be what it seems to be on the surface. This leaves the Year Three Hogwarts students with an exponentially treacherous maze to navigate.
Azkaban is the first of the films where the questions become more internal than external - less the mechanical issues of how do we get out of the Devil's Snare trap or open the Chamber of Secrets, but who can and can't we trust, and why. It's important to note that baddie-in-chief Voldemort does not figure prominently in any respect (his duties are instead relegated to the Dementors, irredeemably nasty soul-suckers that they are). He is referred to, peripherally, by the likes of Peter Pettigrew, aka Wormtail, but we do not see Voldemort in his full flower again until Goblet of Fire. And, in a way, this is appropriate. The transition to the next stage is a knotty and treacherous one for any child at this age, and the absence of Voldemort as a clear and present foil amplifies the uncertainty, leaving the trio of friends largely unmoored from a safe haven (even one defined by the active presence of an enemy) and increasingly in a position where they have to depend on themselves and one another.
Depending on one another, though, is no longer as simple as it once was. Azkaban introduces a heightened level of tension between Harry, Ron and Hermione, often expressed in the form of impatience and temper. You could say that Hermione's time-turner is a metaphor for everything our heroes are going through. They're burning the candle at both ends, literally and figuratively: taking action in multiple places at multiple times, and riding the tide of events that they don't fully understand but are, somewhat unfairly, being required to confront and cope with. The straightforward hero's quest motif of mostly ordered, escalating trials and challenges prevalent in the first two stories is completely upended here into the realm of the unpredictable. Imagine George R.R. Martin (the writer who has given us the world of Game of Thrones) and Agatha Christie teaming up to novelize the labors of Hercules, and you have a beginning...
It's a storytelling evolution that Alfonso Cuarón's directorial style is ideally positioned to mesh with. From the outset and throughout the film, his images are shrouded in mist and in mystery, atmospheric and evocative, with perhaps the only disjointed and endlessly repeated element the slow fades when Harry loses consciousness, a technique that I found lost its impact exponentially with each use, seeming more contrived than in the service of the story. Apart from that one tonal miscue, though, Cuarón captures ideally the driving force behind the Azkaban story: personal relationships and the levels of deception/reception we allow them to encompass.
This comes to a head as Harry finally conjures his essential, game-changing Patronus. Aware that he has nobody to depend on but himself, past or present, he digs down to his essential core and delivers.
At the same time, I don't want to give the many moments of humor in Azkaban short shrift. They range from delights like Harry's Leaky Cauldron stand-off with the Monster Book of Monsters to the almost-slapstick of Lupin's students queuing up to take on the Boggart's cabinet - Snape in drag as Neville's grandmother is worth the price of admission all by itself, as is the Weasley twins' bequeathing of the Marauder's Map to Harry or his confrontation with Snape in the Hogwarts corridor, compelled in an ironic moment to tell Snape exactly what the map - and he - really think! Cuarón evinces brilliant timing in allowing these moments to surface spontaneously amid otherwise tension-filled sequences - think Hermione urging Buckbeak to "come and get the nice dead ferret!" - lending the scene a dark but not hopeless humor appropriate to the story.
Azkaban's final legacy, though, is that it is the film which marks the point at which negative consequences begin to be part of our heroes' adventures. In the first two films, the comeuppances of Quirrell and Lockhart are deservedly earned, and pure justice is served. But in Azkaban, Lupin's resignation is the result of collateral damage and an anti-werewolf whispering campaign. Our trio has taken their first steps into a more adult wizarding world where cause and effect make for an imperfect equation, and nothing is fair. This is a much more sophisticated and complex message that will be brought home smartly in the subsequent installments, both written and cinematic.