PARK CITY, Utah — Benedict Cumberbatch’s latest has all the trappings of a great film: a piece of history few people know about, a chance for a physically transformative bit of acting and, you know, Benedict Cumberbatch.

So, why is it so mediocre?

“Ironbark,” which premiered Friday at the Sundance Film Festival, is a story of quiet heroism and deafening media scandal. It’s about a real-life British businessman, Greville Wynne (Cumberbatch), who was enlisted by the CIA and the UK’s MI6 in 1960 to intercept leaked intelligence from a Soviet source in the guise of innocent work trips. Sell an engine, bring back a microfiche.

The American and British spies — Emily (Rachel Brosnahan) and Dickie (Angus Wright), respectively — dupe the harmless family man into thinking that taking secret missions to the Soviet Union and acting as a mule for information on Nikita Khrushchev’s nukes was perfectly safe. That’s like saying, “Honey, come pet the cuddly alligator!”

But he did it anyway, and formed a lasting bond with the Russian informant, Oleg Penkovsky (Merab Ninidze), code name: Ironbark. Oleg took Greville to his first ballet, got drunk with him on vodka and bought presents for his son — all of which were perilous acts. Together, they helped turn the tide on the Cuban Missile Crisis.

It all sounds fascinating, no? Well, the plot is, even if Tom O’Connor’s script is written without enough concern for fleshed-out characters. The larger issue at hand is that for a movie that whisks us to rooms and outdoor scenes in Moscow, London and Washington, DC, the viewer never feels like he leaves the studio lot. Intimacy is fine, but that’s not what this is. The trouble here is a lack of attention to detail, and laziness with scale.

Director Dominic Cooke’s sense of size is truly wonky. For instance, Brosnahan’s performance as a pushy spy is almost never believable because she’s so publicly aggressive. When Emily and Dickie meet with Oleg about, um, missiles in Cuba, she yaps loudly in a restaurant with little or no discretion. People speak more softly about their credit score. But Cooke has directed not so much a movie as a play.

That makes sense. The director has had a long career onstage, having served as artistic director of London’s Royal Court Theatre for many years. British directors hop between the boards and the screen more often than Americans do. Just look at “1917” Oscar nominee Sam Mendes — he’s had two plays on Broadway in as many years. But the transition isn’t always so seamless.

Cumberbatch, for his part, turns in solid work. He begins unassumingly, just likable enough, and is allowed a splashy sequence of Christian Bale-style acting toward the end of the film. I wanted to worry for his safety more than I did, however, but that’s due to the drama’s consistently slack tension. “Ironbark” is about a moment in history when a worried world collectively held its breath for fear of its own destruction. At the film, we should hold our breath too, for a little while anyway.