When a distinguished actress and an equally distinguished stage director collaborate on a film, one hopes for something special. Unfortunately, in “Red Joan,” the combination of Dame Judi Dench and Trevor Nunn results in something not appreciably better than a mid-level Masterpiece Theatre episode.

The picture is based loosely on the life of Melita Norwood, a British woman who passed along highly classified information about the atomic bomb to the Soviet Union in the late forties, but who was not charged with espionage until 2000. Her story was novelized by Jennie Rooney, and now that fictionalized version serves as the basis for Lindsay Shapero’s screenplay. Perhaps it was the passage of the material through so many stages that led to a film that comes across as surprisingly starchy, stilted and clichéd.

It begins with elderly Joan Stanley (Dench) answering a knock on the door of her London home to find a couple of government agents, who promptly accuse her of violations of the espionage act and place her under arrest. The remainder of the film jumps back and forth between her interrogation and explanatory flashbacks to the events she’s being questioned about. Eventually she will admit her guilt, asking her incredulous son Nick (Ben Miles), a barrister, to represent her in court if necessary.

Back in the late thirties, mousy Joan (played by Sophie Cookson) was studying science at Cambridge when she met flamboyant Sonya (Tereza Srbova), a Jewish refugee from Germany, and her handsome cousin Leo (Tom Hughes), with family roots in Russia. Under their influence, she attended anti-fascist meetings and demonstrations to take action in Spain; she also took up romantically with Leo. Without thinking about it much, she’d become a part of a communist cell, which also included slick William Mitchell (Freddie Gaminara).

With the outbreak of World War II, Joan, with her background in physics, became an assistant to Professor Max Davies (Stephen Campbell Moore), a major figure in a British project to develop an atomic bomb. She accompanied him to Canada when he was assigned to a facility there associated with the Manhattan Project, and on the voyage across the Atlantic they too became lovers despite the fact that he was (unhappily) married.

As the older Joan explains, her political attitudes had developed when Sonya and Leo suffered discrimination because of their views and a tragedy occurred, but it was not until Hiroshima that she decided to transmit atomic secrets to the Soviets. Her motive, she says, was to help create a balance between the two powers and thereby to promote peace—and in the end even Nick, who initially had seemed unable to come to terms with her treason, argues to reporters that her logic appears to have been correct.

Whether or not you’re willing to accept the premise (one the film seems to share with Joan) that the Cold War’s balance of terror—the concept of Mutually Assured Destruction in practice—was key to preventing a renewed world war, the sad fact is that Nunn’s film doesn’t manage to make her emotional journey to that conclusion very convincing from a dramatic perspective. A major cause of the failure is the stiffness of the flashbacks, which under Nunn’s lumpish direction are generally played with too much of the stiff-upper-lip British style that’s become a stale tradition. It doesn’t help matters that the young actors, while attractive enough, often appear to be playing period dress-up rather than creating compelling characters. There’s a good deal on the surface, but one doesn’t feel much going on underneath it (which, nonetheless, looks quite nice, thanks to Cristina Casali’s production design, Charlotte Walter’s costumes and Zac Nicholson’s lustrous cinematography). At least Kristina Hetherington’s editing keeps the back-and-forth chronology smooth, though George Fenton’s score is fairly generic.

Dench is of course the anchor of the “modern” footage, which according to the script began after young Joan’s treason was finally revealed in 2000 as a result of the death of Mitchell. The actress plays fluttery and distracted very well, as you’d expect, but the part doesn’t give her the opportunity to exhibit much range.

There’s a potentially rich story in Melita Norwood’s life that is barely tapped in this tepid refashioning of it. Nunn’s “Joan” may be red, but dramatically it’s definitely not red-hot.