When you turn up to your local multiplex and fork out for a bucket of popcorn and a couple of tickets to Hercules, you’re not going in expecting a cinematic masterpiece. This is, after all, a film about Greek demi-god Hercules with The Rock – sorry, he goes by Dwayne Johnson now – in the lead, directed by Brent fuckin’ Ratner. It’s not the first Hercules of the year (Kellan-Lutz-starring-stinker The Legend of Hercules floated to the surface earlier in the year), and it’s not even the last.

For a while, though, Hercules seems like it’s going to deliver exactly what this sort of film should deliver – delightfully corny entertainment. The opening scene is a perfect introduction to the proceedings, retelling the myth of Hercules in epic, CGI-wrought detail – son of Zeus, conqueror of hydras and so on – before it’s immediately punctured by the reveal that Hercules is merely a talented mercenary; the legend merely a fiction concocted to drive up his asking price. It’s a clever, reflexive take on a tired hero, reinterpreting the genre’s pompous seriousness (see: any part of 300: Rise of an Empire not involving Eva Green) into primary school mythmaking.

Our eponymous hero is introduced emerging from the shadows with a ridiculous lion headdress and a smirk; both pretty much perfect for Johnson, who has the unique ability to elevate trash by simply having a whole lot of fun with it. He’s the modern Schwarzenegger, except where Arnie was at his delivering deadpan one-liners with stone-faced psychopathy, Johnson is most at home when he’s underlining the ridiculousness of the film he’s in. That seems to be the tone for the first half hour or so, though it’s less thanks to our muscle-bound leading man than the contributions of a couple of his compatriots – kohl-eyed Rufus Sewell and soothsayer-with-a-sense-of-humour Ian McShane.

Unfortunately, after those first thirty minutes, Ratner seems to forget that the entire purpose of this enterprise is entertainment. Idle banter and absurd ultraviolence (McShane memorably mows down dozens of enemy soldiers in a chariot with humongous blades strapped to either side, like a macabre ride-on-mower) is replaced with ponderous infantry conflicts and, eventually, a heel-turn twist as perfunctory as it is predictable.

Hercules promises corny mayhem but slides towards seriousness, which does no-one any favours. The PG-13 weightless violence is fine when we’re in Saturday morning cartoon mode, not so much when the script awkwardly tries to emulate Game of Thrones. The clumsy editing, cheap-looking CGI and clunky dialogue become much more apparent when you’re not enjoying yourself – and especially when Johnson isn’t enjoying himself. I can understand why the screenwriters felt the need to introduce stakes and backstory and a whirling moral compass – but why-oh-why did they have to replace Johnson’s grin with a grimace?