It was set in olden times; it was set in modern times. That's the main conceit of Matthew Dunster's new stage adaptation of A Tale of Two Cities — but sadly, it fails to really become a potent tale for anyone's time. Dickens' story of how sacrifice and forgiveness eventually help break cycles of violence and vengeance, set during the French Revolution, becomes a long hard trudge here, not gaining much by being on stage – or by evoking present refugee camps and border control issues between France and England.

There's just so much plot. The whole review could be spent summarising Dickens' improbable but hugely satisfying plot. But the thing is, it's no longer satisfying onstage – it's hard to follow, and requires great lumps of leaden exposition and back story. Information is withheld then revealed, but Dunster's script – and Timothy Sheader's production – fail to make such revelations thrilling; more often, you just wonder if you're missing something.

While they have a laudably diverse cast, I did wonder if the decision to cast a black man and a white man with absolutely zero physical resemble as Charles Darnay and Sydney Carton was more point-making than helpful: the plot rests on the fact that no-one can tell them apart (twice!). Yes, it's a bold move and yes, suspend your disbelief, but in a show already struggling with narrative clarity, it felt obtuse rather than story-serving.

The script is starchy, neither fully aping Dickens nor committing to modern vernacular, and key performances are almost uniformly flat; there's a lot of physical gurning among grotesque aristos and baying mobs alike, but individuals don't get much characterisation.

Fly Davis' set of stacked shipping containers on a revolve is really clever, although experienced technical difficulties on press night – when it works, it allows a slick moving between scenes and locations, and helps set up modern resonances. In the beginning, most of the cast are in scuzzy sportswear; gradually, everyone moves to period garb, before returning to contemporary clothes for the final scenes. A chorus share a microphone to narrate events; Brechtian scene titles and – yawn – contemporary news footage flash on naff little electronic screens which sit oddly amongst the trees at the side of the stage.

It should all feel stylish and punchy, but somehow lacks conviction. Same goes for the stylised approach to showing the monstrous arrogance of French aristocrats: bursts of banging dance music, gold lame leggings and glitter scream excess, but they also scream enjoyable camp. Disco doesn't quite set the scene for the callous death of a small child. The tone is all off.

It's a shame, as the thinking behind it all makes sense: A Tale of Two Cities is an angry story of the one per cent screwing over the masses, and this modern version does newly-mint the novel's concerns with exiles fleeing violence, crossing borders. But even this goes skew-whiff at the end: returning to London from Paris, the Darnay family is split apart. This seems just wrong-headed — they have only escaped together because the character of Carton has made the ultimate sacrifice. If actually they're torn apart at the border, denied entry, was his redemption all in vain? It is a far, far more pointless thing — and a depressing ending to a woeful rewrite.

A Tale of Two Cities runs at Regent's Park Open Air Theatre until 5 August.