Professor Albus Dumbledore (Michael Gambon, left) and Harry Potter (Daniel Radcliffe) return to Hogwarts School of Witchcraft and Wizardry in Harry Potter and the Half-Blood Prince, the sixth film in the series. (Warner Bros. Pictures)

In a freefalling economy, there is a certain comfort to be found in the Harry Potter perpetual promotion machine. Not only have the novels and their spinoffs — movies, DVDs, action figures, T-shirts, lunch boxes, earrings, desk sets and so on — earned billions for their creator, J.K. Rowling, but over the last 12 years, the franchise has helped pay the mortgages of untold numbers of publicists, store clerks, prop builders, toy factory workers and popcorn sellers.

Still, now that Harry Potter and the Deathly Hallows, the final book in the series, has long since come out — and Professor Dumbledore along with it — there is also a certain gamble in prolonging the wait for the remaining three film adaptations. Last year, Warner Bros. bumped the release of Harry Potter and the Half-Blood Prince by six months and announced that the final two movies, which cover part one and two of Deathly Hallows, won't be released until 2010 and 2011.

Rowling's multiple-plot novels are a challenge to adapt. Leave out one spell and you enrage the obsessives; get too arcane, and non-readers will be lost.

All the better for stretching out the revenue stream, but not so great for Harry's earliest fans, who are by now graduating university, while their younger siblings have bypassed Harry altogether to get their fantasy fill from the erotic torment of vampires. And what about the stars of the movies? The protracted run means that they will be firmly in their 20s by the series' end – giving Daniel Radcliffe's onscreen snogging with Ginny Weasley (played by 18-year-old Bonnie Wright) an unfortunate whiff of Wooderson.

And so, at last, The Half-Blood Prince, the sixth film in the series, opens with an unpromising jitter, unsure whether it's kid's entertainment or a grown-up thriller. Picking up where The Order of the Phoenix left off, a gang of Lord Voldemort's Death Eaters, invisible to non-wizards, lay siege to London's Millennium Bridge, demolishing it in a gleeful act of terror. But before that destruction can be digested, Harry (Radcliffe) is whisked off by Professor Dumbledore (Michael Gambon) to help him lure Horace Slughorn, a preening potions teacher, to Hogwarts. Slughorn (Jim Broadbent) holds a crucial piece of information about Voldemort's past that Harry must spend the school year convincing him to reveal. Meanwhile, Harry's longtime adversary Draco Malfoy (Tom Felton) has been recruited by the malevolent one himself to carry out an unknown, but undoubtedly evil, mission.

Rowling's multiple-plot novels have always been a challenge to adapt. Leave out a single spell and you enrage the obsessives; get too arcane, though, and non-readers will be lost. Director David Yates, back again after Phoenix, and veteran Potter screenwriter Steve Kloves deal with it here by throwing in too little of too much. In quick succession, Harry and his pals Hermione (Emma Watson) and Ron (Rupert Grint) pay a visit to a Willie Wonka-ish joke store, spy on Draco getting up to no good with mad-as-a-hatter Bellatrix Lestrange (Helena Bonham Carter), showcasing the nadir of British dentistry, and then hop the slow train to Hogwarts. For all the CGI magic, these early scenes are more kitschy than creepy with little feeling for the peril at hand now that Voldemort is marshalling his supporters.

It's only when the teenaged wizards arrive at the school that the manic pace settles into a suitably ominous gloom. Yates and Kloves are helped immeasurably by the gifted cinematographer Bruno Delbonnel (Amélie), who lights Hogwarts in reds and ochres and composes every frame as if it were a Vermeer. There is one requisite Quidditch match that, at this point in the series, feels like one too many, and a silly little sub-plot about love potions (the Rohypnol of the wizard world). But the best moments are the quietest. An exquisite tracking shot that starts on a heartbroken Hermione (more on that later) being consoled by Harry pulls away to reveal one vignette of lust and another of fear, loneliness and confusion; altogether these three scenes beautifully sum up the emotional terrain of adolescence.

Harry Potter (Daniel Radcliffe, centre), Ron Weasley (Rupert Grint, left) and Hermione Granger (Emma Watson) face dark moments in Harry Potter and the Half-Blood Prince. (Warner Bros. Pictures)

And, after all, this is a story about high school. One of the pleasures of reading Rowling is how well she remembers just how epic the transition from childhood to adulthood feels from the inside, and how neatly she weaves that insight into her fantasy plots. To a teenager, working up the nerve to talk to a first crush is every bit as terrifying as facing a dragon. In the Half-Blood Prince, while Harry pines for Ron's sister Ginny, rational, reasonable Hermione falls even harder for Ron, who, alas, is just not that into her – he's too busy basking in the attention of prattling Lavender Brown (Jessie Cave).

But it's another teenager's coming-of-age that forms the centerpiece of The Half-Blood Prince. Flashbacks reveal the history of Tom Riddle, a.k.a. Lord Voldemort, a brilliant and eerily remorseless kid whom a younger Dumbledore rescues from an orphanage and shelters at Hogwarts. (In a clever bit of casting, 11-year-old Riddle is played by Hero Fiennes Tiffin, nephew of Ralph Fiennes who plays the grown-up Voldemort; 16-year-old Riddle is played by Frank Dillane.) Unlike other teenagers, Riddle is not driven by hormones but a craving for power. Both Tiffin and Dillane play Riddle with a spooky flat affect; in the Muggle world, he'd be the child who tortures cats and grows up to become a serial killer. I won't spoil the plot to reveal what kind of evil a kid like that can get up to when he's versed in magic. Suffice it to say that the film's impressively terrifying climax is straight out of a Japanese horror movie.

As always, the terrific adult cast is relegated to the background. But even in small parts, they shine, particularly Carter as the unhinged Bellatrix, Broadbent as the shallow but ultimately sympathetic Slughorn and Alan Rickman, who speaks volumes with his pauses as the brooding Severus Snape. It's Gambon's Dumbledore, though, who steals the movie. Serving as the setup to the ultimate showdown between Harry and Voldemort in The Deathly Hallows, The Half-Blood Prince, with its standby role in the series, is perhaps the least satisfying of the novels. However, it does mark a poignant shift in Harry and Dumbledore's relationship and Gambon infuses the material with the gravitas of Shakespeare. Dumbledore is the closest person to a father that Harry has known, and there's a sacrifice that Dumbledore requests of Harry that finally, irrevocably, transforms the boy wizard into a man.

Harry Potter and the Half-Blood Prince opens on July 15.

Rachel Giese is a Toronto writer and editor