Red Joan is a spy thriller that mostly lacks thrills. It takes the not unusual trick of building the story around flashbacks, but in this case, the structure doesn't really do anything for the movie.
Judi Dench is an elderly woman arrested for being a spy. During interrogations, we get her story, with Sophie Cookson playing the young "Red Joan". The scenes of the past are engaging enough, and the arc whereby Joan becomes "Red" is fairly engrossing. But whenever the movie returns to Dench, all the momentum dissipates. At some point, I realized the entire movie could have been made without Dench, without the scenes in the present, and been just as good, just as intelligible. Better, in fact.
This is not to say that the flashback structure is never a good idea. But Cookson has an engaging flair, and while it's not Dench's fault, the old Joan is miserable and depressed. It makes sense for the characters, but the depressed Joan is never compelling enough that I don't wish to return to Cookson.
Red Joan is competent, and it will satisfy those looking to pass a couple of hours. But that's about all it is.