Red Joan, don’t make it bad, take a sad song and make it better. Or, if you can’t – take the real life story of a Cambridge Spy who supplied atomic bomb secrets to the Russians in World War Two and fictionalise it, but in the most perfunctory and sentimental way possible.

Why, you ask, when you have the story of the grandmother spy, Melita Norwood – why not just dramatize that? Was the real story too complexed, too nuanced? Was it missing that special ingredient, beloved of movie audiences – namely sympathy for, and identification with, the main protagonist? Was Melita just a bit of a bitch who turned a blind eye to atrocities in the Stalin regime?

At least when Dennis Potter had a go at this sort of material, in his Play for Today, “Traitor” – he had the balls to make his Soviet agent in the foreign office a true believer; someone who’d been duped by the communist critique of the West. Sympathy for him came from knowing that a) he’d been brutalised by the British class system, so had developed a seething hatred and disdain for the establishment and b) was only now realising the extent of his mistake as he rotted in a cold Moscow apartment block.

Red Joan doesn’t quite have the Engels to make its fictional protagonist a Stalinist convert. The promise of the premise is close identification with a young woman, recruited at Cambridge, and pulled into the orbit of Soviet sympathy; a potentially relevant story in these politically confused times, when young people are being seduced by the politics of populism and demagoguery.

Instead, the film patronises its audience by having Sophie Cookson’s sweet and pretty Joan fall for the revolutionary on campus – a cold German émigré, spouting clichés. She’s convinced by him, and his political circle of friends, who envelop her; creeps that hound her wherever she goes (a sign they may not be interested in her company alone). In time she comes to believe that Stalin, despite those rumours of political persecution (read: murder), should have the bomb as a deterrent against Western aggression, thereby securing the peace.

“I was right though, wasn’t I?” a defiant Judi Dench screams at her barrister son, when the depth of her complicity comes to light fifty years later. Well, one doesn’t like to say that Russia nearly started a nuclear war by accident twice – the first time in 1983, and the second during Boris Yeltsin’s stint as president, or that Russia’s nuclear capability has licenced it to oppress its people, and those it deems to be its people, without fear of retaliation, but yes, the peace has held Joan – well done you for robbing the democratic powers of their short-lived military advantage in a world encircled by reactionary, oppressive forces.

Ultimately, Red Joan is a dull and plodding tale of a gullible woman and the men she ruined, apparently motivated by world peace. Trevor Nunn gets committed theatrical performances from the cast, as you’d expect, but as you’ve also anticipated, adds nothing approaching tension or style to what is allegedly a cinematic endeavour.

As this is a fictional account of spycraft, with invented characters filling in for their real world proxies, there was the opportunity to better tease Joan’s guilt or innocence, and muddy the water of her political sympathies as she sank deeper into the Soviet mire. Instead, Nunn does very little with the story – content to offer a period melodrama capped with a piece of heart tugging but trite and naïve moralising. Better dead than Red Joan.