“ Black Bag ,” an elegant, quick-witted new thriller that finds Cate Blanchett and Michael Fassbender playing married spies, isn’t the pair’s first time together on screen. That honor goes to 2017’s “Song to Song,” one of Terrence Malick’s most intangible and head-scratching offerings — though the two stars can barely recall that experience.

Black Bag centers on employees at the London-headquartered National Cyber Security Centre, leading some fans to wonder if the film is based on a true story.