We're going to have to start seriously asking if Dexter Fletcher is a genius. I remember being charmed by his debut Wild Bill, and I enjoyed its follow-up Sunshine on Leith while also wishing someone had given him enough money to make the big, unapologetic, MGM-style musical he obviously wanted to make. Now someone has, and it's little short of spellbinding.

Fletcher's last directorial credit was Eddie the Eagle, but his last directorial work was as an emergency nonce replacement on Bohemian Rhapsody. I freely admit that I probably overrated that film, and it's not like I don't understand the arguments against it. But a lot of what I do remember fondly from it is in here, as well; the near-archaeological attention to the furnishings of mid-century working-class British homes, the knowledge that a stage persona can be both a disguise and an autobiography. Bohemian Rhapsody argues counterintuitively that becoming Freddie Mercury liberated Farooq Bulsara. Rocketman, on the other hand, documents the ways in which Elton John devoured Reginald Dwight. Yet after two hours of pilled-up, extravagantly costumed, vodka-for-breakfast excess, it also persuades you that the child is still crucially important to the man.

But the comparison is as superficial as the Flash Gordon-via-Dame Edna stage costumes that Elton uses to hide his stage fright. Rocketman is not a biopic, it's a musical, a full-on, fantastical one where every time someone sings crowds of dancers burst out of nowhere. The standard rise-fall-rise narrative of a rock biopic is so obviously refreshed by this genre shift it's a wonder nobody's done it like this before. You don't have to sit through awkward scenes of actors pretending to come up with melodies we already know by heart when they can just jump on a table and start singing the damn songs.

Jamie Bell broke through playing a dancer but it's still remarkable how well he acquits himself in a proper musical, and Bryce Dallas Howard turns in an utterly transformative performance as Reg's wayward mother. It's 100% Taron Egerton's show, though, and he aces it. He's done drama and comedy before, and sang in the animated film, er, Sing. None of this prepares you for what a natural showman he is, though, slinking through 'Honky Cat' and 'I'm Still Standing' with a musicality to his walk that a mere actor-who-sings could never manage. There's a terrific moment where Elton is at his bombed-out worst and he sings 'Sorry Seems to Be the Hardest Word'. His body language is convincingly drunken and depressed but his voice is clear as a bell. It's astonishing that an actor born so long after the golden age of Hollywood musicals should understand so innately the level of unreality they should be pitched at.

The same goes for Fletcher. People who know me might be forgiven for thinking I don't like musicals, but that's not true; I'm just incredibly choosy about them, far more than I am for any other film form. If it's not Bob Fosse or Stephen Sondheim I get worried. This isn't because I think it's a bad genre, it's because I see it as an extraordinarily sophisticated genre with complexities I'm nowhere near understanding. I can only say that by the second number - 'I Want Love' - I knew Fletcher and his screenwriter Lee Hall completely got how a musical number can group together disparate characters, how it can propel the plot by venting emotions that might otherwise be relegated to subtext.

The number that follows it, a dizzying Steadicam whirl through pubs and fairgrounds set to 'Saturday Night's Alright for Fighting', is even better, adding a cinematic literacy with its unmistakable nod to that lodestar of British rock musicals, Val Guest's Expresso Bongo. It's one of a series of nods to film musicals past that never sinks into slavish imitation. The long, fluid takes and full-body framing suggests Fletcher is a fan of Stanley Donen, the mixture of fantasy and autobiography recalls Bob Fosse, and there's even a number in an LA private clinic that suggests an infinitely more glamorous remake of The Singing Detective. Ken Russell, the director of Elton John's one memorable film performance, is inevitably invoked, and the 'I Want Love' number includes a visual cue to Sam Taylor-Johnson's iconic video for that song. There are also plenty of less obvious cues, like an eye-popping 'Bennie and the Jets' which resembles The Lost Weekend reincarnated as the video to 'Fast Slow Disco' by St. Vincent.

Holes can be picked here and there. There is a fleeting mention of 1980s tabloid stories about Elton that gives the misleading impression they were accurate, rather than genuinely demented homophobic bile, but I guess you can't expect a film produced by Matthew Vaughn to be critical of The Sun. Mostly, though, this is a Herculean effort, turning a premise that could so easily have been embarrassing, or clever-clever, or simply clumsily done into spellbinding, joyous, emotional pure entertainment. And this is coming from someone who isn't even a particularly big Elton John fan, so imagine how much of a wreck I'll be when Fletcher makes Jarvis!: The Musical.