When the sixth novel in the Harry Potter series was released in July 2005, it sold nine million copies on the first day of publication.

Its main purpose is to set up the final conflict between Harry Potter and his chief antagonist Lord Voldemort, which will take place in Harry Potter and the Deathly Hallows, currently being filmed in the two parts that will conclude the saga. On film, part of the joy of Harry Potter has been watching him grow up and grow into his wizard’s character, which requires him to confront the legacy of trouble haunting his young life. The present film is a stepping stone, neither classic Potter nor scintillating adventure, but it proves the depth and wholeness of Rowling’s imagined world. It is a world that seems increasingly more charming — more dangerously charming — than that of Tolkien or CS Lewis.

Harry (Daniel Radcliffe) is now in his sixth year at Hogwarts and it seems the school is now Crushville, where every pupil has the hots for somebody. Ron Weasley (Rupert Grint) is doing particularly well, giving a big-up to gingers everywhere and by the end of the film he looks fairly dizzy with snogs. The death-eaters are causing havoc in both the Muggle world and in the magical one: the film opens with a brilliant sequence showing them swooping down on central London, popping the rivets of the Millennium Bridge and dashing it into the mucky Thames. Not a second is wasted in this first half-hour of the film. You feel that the richness of both worlds might have reached a pinnacle of interest, with exciting stuff happening wherever you look.

Accompanied by Albus Dumbledore — the bumbling, weary headmaster of Hogwarts, played with timid authority by Michael Gambon — Harry goes in search of an old teacher who must come back to the school as Head of Potions now that Snape (Alan Rickman) has moved on to become a Defence Against the Black Arts instructor. In a sleepy village they find their man, Horace Slughorn, masquerading as an armchair in a dilapidated house. It is an immediate pleasure to see Jim Broadbent as Slughorn: right away he suggests gentle rule-breaking, cheerful experience and before long we see him back at the school, inviting Harry to the “Slug Club”, a gathering of favoured students who come to dinner in Slughorn’s rooms. In the potions class, Harry is allowed to borrow a textbook that once belonged to “the half-blood prince”.

The book brings inordinate success and Harry uses it to gain a “memory” from Slughorn about his old pupil Tom Riddle (Voldemort as a sixth-year boy), a memory Dumbledore was keen for him to get, and so we find out more about what constitutes the nature of the Dark Lord’s immortality. Dumbledore has taken Harry in hand and has been showing him a collection of such “memories” to help him figure out the secret of Voldemort’s power. We know that Voldemort’s soul is split into seven pieces and that six of them are attached to magical objects called “Horcruxes”, which are identified for the first time here. It is the fight to destroy the horcruxes that will drive Harry’s final confrontation with the Dark Lord. This film shows, with a central dramatic surprise, how costly the attempt to defeat evil is going to be.

The central question here is whether you can control fate by understanding the past. Yet the action goes a bit spongy in the middle, a bit soggy with underpowered events, and the large audience I saw it with became restless as they waited for a payload of magic. The Potter-world is so dense that the adapter must struggle to achieve a balance between the soap opera of snogs and the gathering storm of Harry’s battle with Voldemort. The strain often shows here and I feel director David Yates goes for too much of the former, making the story work in television terms (a forte of Yates’s: he’s a veteran of The Bill and the wonderful TV drama State of Play) while letting too much wonder seep out of the episode.

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The film is a strike for realism pitted against fantasy, but in a Harry Potter film that is likely to feel an empty victory.

I thought often of the Charles Dickens franchise while watching the latest Potter. The comparison is too seldom made but the beautiful characters, to say nothing of fate’s entanglement with the past, allow you to feel that the Potter stories are nicely congested with ingredients that may be satisfying to those brought up on the work of the great comic realist. Whether sleazing around Hogwarts, running up and down the seven floors of The Burrow, the Weasley home, or negotiating the old cobbled streets and country winds of the Muggle world, Harry Potter is really a wizardly David Copperfield (whom Radcliffe once played) and in the Half-Blood Prince we begin to understand what the past has prepared him to do.

It is noticeable how epic and dark the series has become since those early days of cupboards in Privet Drive and speeding red buses. The black cloud of the death-eaters is never far away and, in this film, we see young Draco Malfoy (Tom Felton) growing more and more sinister. Daniel Radcliffe has grown marvellous in the central part over the years: he has the same lucky grin, the same pent-up sadness, but in The Half-Blood Prince he begins to don, most believably, the mantle of destiny. It was always part of the boy’s charm that he couldn’t see himself as a leader, but during this latest instalment he begins to look like one, despite himself — and so the film, though patchy in itself, has done its work in preparing us and Harry for the great battle ahead.