The strangest thing about the new Harry Potter movie is not that it’s unusually good, which it is, but that it unequivocally illustrates just how poorly we’ve been served by the previous five instalments in the franchise. Here, as willing consumers of a £2 billion brand, we’ve swallowed wooden performances from adolescent actors, grossly broad caricatures from seasoned thesps, ramshackle plotting and sententious Manichean drivel all naffly coated in pseudo-Latinate esoterica (“Expecto Patronum!” etc).

That’s all changed, however, with Harry Potter and the Half-Blood Prince. The movie plays fast and loose with J. K. Rowling’s overstuffed source novel, and excises several needless flashback sequences while controversially adding an opening gobsmacker of its own — namely, the destruction of London’s Millennium Bridge by a team of ghastly Death Eaters (cohorts of Ralph Fiennes’s über-villain Voldemort). From here, the director David Yates calls an immediate cessation to all CGI showboating and concentrates instead, for almost the entirety of the second act, on the torrid emotional lives of the Hogwarts teen triumvirate, Harry (Daniel Radcliffe), Hermione (Emma Watson) and Ron (Rupert Grint).

Thus, like a toned-down episode of Skins or a sexed-up version of Grange Hill, the Potter party are pinged through a series of hormonal highs. At dinner parties, post-Quidditch parties and Christmas parties the confused and unrequited love that Hermione feels for Ron is sorely tested, while Harry receives his first kiss from Ginny Weasley (Bonnie Wright) in a delicately executed scene that is emotive enough to give quiet goosebumps. In this the junior cast, for the first time, do “proper acting”. Radcliffe, perhaps emboldened by his stage time in Equus, has leavened his formerly plank-like turn with smart self-deprecating shrugs and sighs, while Grint especially is becoming something of an accomplished comic actor.

The meta-plot — the light versus dark shenanigans — eventually lurches into gear sometime in Act III. But even then it’s not the traditional Rowling tease, but a substantial encounter with the enemy that results in the desecration of Hogwarts and the death of a major character (plus a fantastically cheeky homage to Brian De Palma’s Carrie). There are, of course, only two movies left in the series (Harry Potter and the Deathly Hallows, parts 1 and 2). But on the evidence of The Half-Blood Prince there is a real chance that Potter, like The Lord of the Rings before it, will be a fantasy franchise that ends in a bang, not a whimper.

12A, 153mins

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