Starring Judi Dench. Rated PG

Near the start of this pleasantly old-fashioned spy story, a prim young physics student slips into blood-red heels left behind by a more glamorous coed and, for a moment, she’s not in Cambridge anymore.

Young Joan (Sophie Cookson, of the Kingsman movies) doesn’t quite dream of flying over the rainbow. She just wants to break into the all-male science world of 1938. Then she meets Russian-born Sonya (Czech Tereza Srbova), of the aforementioned ruby slippers, and Sonya’s cousin Leo (Tom Hughes), a tousle-haired bad boy with espionage eyes.

With Spain in flames and another world war looming, the cousins want to help their home country in the worst way. They get their chance when Joan graduates to a top-secret program rivalling the Manhattan Project to develop a bigger bang for the Allies. She’s not hot on their Soviet sympathies, but we know she’ll divulge some atomic secrets, because the very first scene tells us Joan (or Cookson) will eventually morph into Dame Judi Dench and get arrested, in 2000, for her retroactive sins.

Perhaps because she’s busy bossing James Bond around, Dench isn’t on-screen that much. But since you need a national treasure to sell a small-scale period piece like this, screenwriter Lindsay Shapero and director Trevor Nunn (better known for his Shakespeare work on-stage) fashion all their back story around the arrest. This setup makes Britain’s longest-hidden Cold War snitch tell her story to what seem like ordinary police, whose station-house questions seem more expository than MI-6–ish. “Is that when you first met your husband?” for example, as opposed to “How exactly did you get those damned schematics out of the country?”

Red Joan is based loosely on the real-life Melita Norwood (now, there’s a spy name), a secretary, not a scientist, and a staunch Communist who didn’t need romantic entanglements to prompt her betrayal. Instead, the filmmakers hang nonsaint Joan’s whole character arc on her apparent need to see nuclear technology shared equally—pretty flimsy, philosophically, and even thinner from a storytelling angle. If Nunn and Shapero were going to change almost everything about their subject, they could have come up with a lot twistier shit than this. Though handsomely staged and shot, the movie lacks urgency and thrills, and while the actors are adequate, they all seem strangely enervated by the task of pretending to be people they are not.