For rabid fans of a certain bespectacled boy wizard, July 21, 2007 was both a blessing and a curse. That was the day Harry Potter and the Deathly Hallows, the seventh and last book in JK Rowling’s wildly successful fantasy series, was released worldwide. Though Potter-philes finally learned whether their hero would defeat the evil Lord Voldemort or die trying, they also had to come to terms with the fact that Harry’s adventures were over – at least, unless Rowling decides to upgrade herself from unbelievably rich to disgustingly wealthy by penning another magical tome.

In the meantime, we’ll all have to make do with Warner Brothers’ film adaptations of the Potter books. Don’t get me wrong: the movies have plenty of their own merits. Their gasp-inducing visual effects are second to none, and the series has always featured an impressive Who’s Who of famous British actors: Maggie Smith, Kenneth Branagh and Emma Thompson have all gamely donned robes and wands in various instalments.

Still, it’s inevitable that something will be lost in translation when a story is moved from page to screen, especially if the source material is as rich and layered as Rowling’s books. (Stifle your scoffing: Harry Potter may have begun as a set of books for children, but as JK’s writing became darker, more complex and altogether better, the series developed into something that’s gripping regardless of how old you are.)

The perils of adaptation are more apparent than ever in Harry Potter and the Half-Blood Prince. While its literary analogue, the penultimate Potter tale, is arguably Rowling’s best novel – fast-paced and exciting, with the most gut-wrenching ending in the entire series – the movie version leaves much to be desired.

Somehow, director David Yates has managed to create a film that will both baffle viewers unfamiliar with the magical world of Harry Potter and irritate others who know and love the books. Half-Blood Prince contains little to no exposition, so anyone who doesn’t understand why Harry reacts a certain way when he hears the name Sirius Black will have no hope of understanding anything that happens over the next two and a half hours.

On the flip side, fans will be scandalized to find that screenwriter Steve Kloves, in an attempt to condense the story, has mistakenly removed most of Half-Blood Prince’s narrative backbone: a series of flashbacks that explore Voldemort’s history. While the book adds much-needed complexity to a potentially one-dimensional villain by explaining in detail how he became an evil mastermind, the film features just two digressions into the bad guy’s past.

Those scenes, in which young Voldemort is played by Hero Fiennes-Tiffin – nephew of Ralph Fiennes, who plays the grown-up Dark Lord – and Frank Dillane, are tantalizing enough to imply that Half-Blood Prince may have been great if it had focused on Harry’s nemesis as much as the book does. Eleven-year-old Fiennes-Tiffin in particular is terrific; he’s the creepiest kid to hit the silver screen since the twins from The Shining.

But because those scenes are few and far between, the latest Harry Potter film feels shapeless and unfocused. The plot, such as it is, follows Harry’s (Daniel Radcliffe) attempts to get close to weak-willed Potions professor Horace Slughorn (Jim Broadbent), who once taught the young Voldemort. Harry has to persuade Slughorn to give him crucial information that could contain the secret to destroying Voldemort once and for all. Along the way, Harry and his best buddies, Ron (Rupert Grint) and Hermione (Emma Watson), must deal with an enemy even more potent than the Dark Lord: their raging teenage hormones.

See, Harry likes Ginny (Bonnie Wright), Ron’s sister, but Ginny has a boyfriend. Hermione likes Ron, but Ron is dating newcomer Lavender Brown (Jessie Cave). Romantic entanglements take up far more screen time than anything else in Half-Blood Prince, which gives the film a welcome jolt of levity – past Potter movies, especially Harry Potter and the Order of the Phoenix, have been overly gloomy. Most of the laugh-out-loud moments in this film involve Lavender, the obsessive, hilarious first-girlfriend-from-hell.

Still, focusing on young love rather than analyzing Voldemort means little sense of urgency in the Half Blood Prince. With this film, the franchise should be setting viewers up for the big finish that will come with the adaptation of Deathly Hallows – or, rather, adaptations, since the studio is splitting the book into two movies.

Instead, the movie’s ending, which should be a traumatic game-changer, falls totally flat, perhaps because a climactic battle scene from the book has been excised. A diehard Harry fan – or anyone who enjoys compelling cinema – can only hope that the decision to stretch book seven into a pair of movies is, at least partially, artistically motivated. Perhaps this approach will lead to a couple of movies that can please fans as much as they hook casual viewers. Until then, films like Half-Blood Prince – entertaining, sure, but not as satisfying as the novels – will have to be good enough.