The great Dame Judi Dench has all but retired from acting, her eyesight not allowing her to memorize scripts the way she once did. So so you’d better take care to use this resource wisely, surrounding her with stellar supporting players and a worthy film.

“Red Joan” is a run of the mill espionage thriller, another period piece spy tale about those young, educated British idealists who sold out the West to Stalinist Russia at the very beginning of the Cold War.

Dench plays a spy uncovered decades after the fact, a “Cambridge Spy Ring” member caught and interrogated in her dotage, remembering the romance, recruitment, the ideological debates during those heady days when the world teetered on the brink of fascist conquest.

Dench’s is the first face we see, a little old lady in a row house, tending her shrubs, noting a government figure’s death in the news and abruptly arrested that same day. “Red Joan” is framed within her interrogation, her flashbacks taking us back to a time when “The world was so different, then. You have no idea.”

Present day Joan may be stonewalling her interrogators, and even her unknowing barrister son (Ben Miles). But we see it all, in depressingly familiar and melodramatic story beats.

Young Joan (Sophie Cookson of the “Kingsman” movies) was studying physics in Cambridge in 1938 when she fell under the influence of Sonya (Tereza Srbova), glamour puss bad girl on campus, a Jewish emigre who ducks into her room and invites her to a communist meeting where they show Eisenstein propaganda films.

“Everybody did it back then,” she explains to the intelligence officers questioning her. “It was the ‘in thing.’

In her memories, she was skeptical to the point of cynical about Stalin and Stalinists. But in Sonya’s “cousin” Leo (Tom Hughes), a compelling speaker and a true believer, Joan becomes “Jo Jo,” and is at least willing to hear his arguments.

Not when he brazenly suggests she steal secrets for “our Allies, the Russians.”

He’s preached communist idealism, “the chance to rebuild civilization from scratch” while she was going as far as a woman could in the scientific ferment of the race to build “The Super Bomb” during World War II. Her education put her in the charge of a researcher (Stephen Campbell Moore) in the front lines of the research.

The novelty here is that is no novelty at all in packaging this as a love triangle, the “bad boy” spy, vs. the unhappily married scientist, competing for Joan’s affections and ideology.

Joan struggles with that in the past and with her son’s disillusion in the present, rejecting entreaties to “Follow where Stalin leads” and “saving the Revolution” but somehow jolted into abandoning those doubts when America acquires the bomb first.

“It’s like I don’t know you,” is one of many pedestrian lines the cast must deliver (by Miles). Only Moore’s Max, the scientist, has anything poetic to say, inviting her into the secret world of A-Bomb research “break the machine, touch the ghost of matter.”

Dench gives a knotty, empathetic performance, reluctantly self-righteous. And Crookson is a perfectly serviceable, fiery younger Joan. The men? They’re just archetypes, and rather drab ones.

The script’s one clever touch is jabbing at the sexism of the times, the patronizing way even a woman of science could be treated by the war machine of the 1940s. Not that this motivates Joan, who is based on Britain’s “Granny Spy,” Melita Stedman Norwood.

If the film does her and her generation any service, it is in bringing up the context of the times, the idealism and lack of hindsight that today we treat as naivete. No, she and the Cambridge Spy Ring don’t get a pass. Stalin’s depredations were pretty widely known and there really was no excuse for swallowing agitprop from a good-looking recruiter.

But “Red Joan” doesn’t do much with this promising story, content to skim the surface and accept Joan’s as the only narration or version of events worth accepting at face value.

That doesn’t do her, her crimes or Dame Judi justice.

MPAA Rating: R for brief sexuality/nudity

Cast: Judi Dench, Sophie Cookson, Laurence Spellman, Ben Miles, Tereza Srbova and Stephen Campbell Moore.

Credits: Directed by Trevor Nunn, script by Lindsay Shapero.  An IFC release.

Running time: 1:41