One of the problems with the spate of serial movies that have cropped up in recent years is how to deal with the difficult middle instalments. 'Lord of the Rings: The Two Towers', 'The Matrix Reloaded' and 'Pirates of the Caribbean: Dead Man's Chest' all suffered from a severe self-imposed narrative handicap - they existed in a weird limbo where characters had been already introduced and themes were to be advanced but where resolution could not be supplied. At best these movies were limited to the development of characters, setting the scene for a big finale. At worst they served as extended prologues. All three were guilty of employing big set-pieces and special effect extravaganzas that had only the flimsiest narrative purpose.

The Harry Potter films have by and large avoided this by merit of their basis on self-contained novels: part of a series but not slave to that overarching narrative (as opposed to Tolkein's original 'Lord of the Rings'). However, in book form 'Half-Blood Prince' is arguably little more than an extended lead-in to the big climax of 'Deathly Hallows', and the cinematic adaptation emphasises this.

Indeed there is no easy way to summarise a film which has no narrative thread of its own. Evil Voldemort is out there somewhere but we see nothing of him during this movie. The focus of the villains' machinations is the removal of a key figure, but this remains unexplained until the final act. The heroes don't have any plan or mission as such - their entire achievement in the course of a 150 minute film is to find out something about Voldemort. No plots are foiled, no disasters averted. It all seems a bit hollow.

That said, the inheritance of these shortcomings from the book has allowed the film to develop in other ways. Free of the need to dash from supernatural set-piece to spectral wizard-battle the young actors of the film have finally been given chance to, well, act. There's a sense that, after eight years of playing the same characters Daniel Radcliffe, Emma Watson and Rupert Grint are having some fun with them. By now Harry, Ron and Hermione are in early adolescence and while there is no grit to their teenage years (it's Hogwarts, not 'Grange Hill') there's a bit bit more depth to the performances, a little more maturity to the acting, a sense of chemistry between all three that had largely been lacking up to this point (Grint especially has a likeably goofy style that is not a million miles away from Simon Pegg). Couple these with some more fine turns from veteran British actors such as Alan Rickman, Michael Gambon and, all too briefly, David Thewlis. Best of the bunch is Jim Broadbent, perhaps the most versatile British actor around at the moment, who manages a performance that is, in equal part, sinister, bumbling, condescending and obsequious.

What is more 'Half-Blood Prince' does not call upon noise and thunder to try and conceal its narrative short-comings as the various middle-instalments mentioned above do. It is perhaps the Harry Potter film least reliant on special effects - they are ubiquitous but do not take centre-stage. Deprived of dazzling visuals and ruthless narrative imperative, the amazing thing is that 'Half-Blood Prince' just about gets by on its characters alone. This is more than can be said for most of its spiritual predecessors.

Stephen Lavington is the author of a critical guide to the work of Oliver Stone.