Two words sum up the experience of watching Harry Potter and the Half-Blood Prince. Dull and disappointing. More so than any of its predecessors, this sixth instalment in the insanely popular series about the boy wizard has suffered from the compression of a sprawling work of fiction - some 600 plus pages of multiple interwoven plot lines - into two and a half hours of screen time.

I say compression and not adaptation because very little of the novel's substance has found its way to the screen. So little in fact that one gets the impression that screenwriter Steve Kloves was so daunted by the amount of material covered in the novel and the almost impossible task of adapting it to a single movie, that he threw away his quill, picked up his wand and cast a reducto spell on it. All that remains of Rowling's mammoth novel is a little dust.

To be fair, it's hard to lay blame with Kloves alone as the undertaking was fundamentally flawed. Two and a half hours screen time is simply inadequate to do justice to the material. In the previous two movies in particular (Goblet of Fire and Order of the Phoenix) there has been an increasing dependence by the filmmakers on the audience's familiarity with the novels to fill in the gaping holes in the movie's version of the story, but Half-Blood Prince takes this to such ridiculous depths that it feels downright lazy.

This is all the more disappointing because the novels and the movies are otherwise increasingly interesting as the children grow into young adults and the challenges they face become darker, more perilous and, well, more adult. The Half-Blood Prince is a ripping novel with multiple engaging plot lines. In particular its painstaking unearthing of Voldemort's past and the long overdue development of Snape's place in the scheme of things are highlights. While the movie is certainly more grim than its predecessors, this is more atmosphere than substance and very little of the material about Voldemort and Snape finds its way to the screen. What we do have in great and tedious detail is a focus on the adolescent relationships of Harry, Ron and Hermione. While this is one aspect of the novel - and in its place it is engaging and humorous - it's as if the producers have sacrificed the integrity of the story and chosen instead to exploit the undeniable charm and real-world popularity of the franchise's young stars and the adolescent target audience's hormone-fuelled fantasies.

Nonetheless, and within the tight constraints imposed by the script, the young actors do an admirable job. Jim Broadbent is also perfectly cast as the affected Professor Horace Slughorn and his is without doubt the standout performance; unfortunately we don't see nearly enough of Helena Bonham Carter as the maniacal Bellatrix Lestrange nor, as mentioned, of Alan Rickman's Snape, but gone is the hint of a smirk in his depiction of the professor and the great tragedy of his life shines darkly through. In the end, my disappointment with the movie is all the more intense because the performances, which promise so much, are denied an opportunity to take flight.

All is not lost however. As is well known, the final novel, Harry Potter and the Deathly Hallows, will be split into two parts for filming, with Part One scheduled for release in November 2010, and Part Two in July 2011. Hopefully this will give director David Yates (responsible for Half-Blood Prince and Order of the Phoenix) and Steve Kloves the opportunity to do justice to the novel. Kloves had this to say about the split: "I'm really excited about it because it should allow us to stretch a bit with the characters and give them the proper send-off. The story is highly emotional and those moments deserve time to breathe. And, personally, I feel we owe it to Jo — in order to preserve the integrity of the work — and the fans — for their loyalty all these years — to give them the best and most complete experience possible."

Let's hope they pull it off.