If there has ever been a movie with greater pre-release fanboy goodwill than Star Wars: Episode III – Revenge of the Sith enjoyed, it is hard to remember it. George Lucas’s epic finale to his misguided prequel trilogy about Anakin Skywalker’s transformation into Darth Vader was released in cinemas 15 years ago this week, and it is still possible to recall the palpable sense from acolytes of the space saga that this time ... surely, this time ... they would get it right.

We really should have known better. Episodes I and II had already come and gone over the previous six years with the highs of pre-release buzz followed by the lows of crushing disappointment. Why should Revenge of the Sith be any different? Still, the whole point of Star Wars is blind faith in the cosmic divine, so you can forgive us for holding on to a little bit of hope.

Part of the reason Revenge of the Sith summoned up fresh reserves of positive thinking was the subject matter. At some point, we were going to see Hayden Christensen’s Anakin actually becoming Darth Vader, with all the attendant, juicy horrors of this final, grisly metamorphosis. It was hard to imagine the kid-friendly veneer of The Phantom Menace being transferred to a sequel with such a dark and torrid story – a once bright-eyed young Jedi devolving into a hideous, black-carapaced death cyborg. There also ought to have been a bit more actually going on than in the glacially paced Attack of the Clones, a placeholder of a movie if there ever was one.

I can’t remember if we already knew, prior to release, that Ewan McGregor’s Obi-Wan Kenobi would be the one to maim his former young apprentice and leave him burning in agony on the lava-strewn shores of Mustafar, but everyone had a pretty good idea there would be a climactic Jedi battle between former allies at some point. Finally some genuine action and an escape from the previous episode’s obsession with the galactic senate, which made watching Clones a soporific experience – like tuning into Star Wars’ equivalent of the BBC Parliament channel.

In the end, it’s this acceleration of the stakes that makes Revenge of the Sith the best of the prequel episodes, even if this is far from a recommendation and there are still far too many scenes in the senate. The Mustafar scene is brilliantly memorable, and Anakin’s grooming by Ian McDiarmid’s Palpatine/Darth Sidious is satisfying enough, thanks to the Scottish actor’s ability to flip from elegant Jekyll to furniture-chewing Hyde in the flash of a lightsaber. There is a reason McGregor is coming back as Kenobi for a TV show set before the events of the original 1977 Star Wars: throughout Revenge of the Sith and earlier prequels he is a rock of quiet charisma. To retain enough Star Wars gravitas that fans can’t wait to see you return as the Jedi master, despite having appeared as the same character in three deeply flawed movies, is quite something.

Unfortunately, there is also a reason why Christensen’s career has nosedived since 2005. He was the weak link that sank the whole trilogy. The tale of Anakin’s transformation into Vader, on paper, has the scale of Shakespearean tragedy, yet watching the prequels is a bit like travelling to Stratford-on-Avon in anticipation of world-class theatre to find a daytime TV star playing the lead. Instead of eliciting horror and awe, the final scene in which we see what remains of Anakin’s ravaged torso encased in the infamous “mini-Darth armour” evokes only scorn. It is impossible to believe that this callow whinger has any relation to the iconic villain of the original trilogy.

There are dozens of other reasons why Revenge of the Sith doesn’t pass muster, of course – Natalie Portman’s inability to find any sort of chemistry with Christensen, Lucas’s obsession with sterile CGI, the dreary staleness of the Jedi Council – but he is the nail in the coffin. That a film-maker who managed to discover Mark Hamill, Carrie Fisher and Harrison Ford chose this guy to lead the final two episodes in his prequel triptych tells you everything you need to know about what’s become of Star Wars’ creator.