As a whole, I feel that it is hard to judge the accuracy of any spying operation presented in any media, as having a basis to judge accuracy kind of defeats the basic purpose of the operation. However, Operation Mincemeat I believe could be counted as a fairly accurate portrayal of spywork and espionage, with its roots in an actual and fairly well-documented operation conducted by MI-5 during World War II.
The show tells a very fun and unique spin on the common spy story, steering itself more towards historical accuracy than dramatization, which itself is oftentimes stranger than what fiction could have been. Through this more historic lens, the show presents a far more sequestered and distant form of spying than the upfront, action thriller something like a James Bond movie would showcase. It portrays espionage work as something done more like a NASA rocket launch, with many people working for long periods of time to make an infallible plan, and having no way to ensure its success after sending it off other than a hope and a prayer. While one would assume that an indirect involvement with the plan would lower the stakes to the audience, as the characters are not the ones directly in danger, the stakes are truly intensified with this presentation, as it makes every small error and unaccounted for situation when planning and creating that much more important, as there is nothing they can do to rectify the situation if it does go wrong. Even some of the show’s more ridiculous or seemingly fictional elements added to increase drama and emotional impact to the story were actually real things about this operation, such as Hester Legatt’s song “Dear Bill” in Act One (utterly fantastic song, by the way) being based off of a real letter that the actual Hester wrote during World War I, or that Ewen Montagu did have a filmmaker brother who was also a communist spy (he was also a Hall of Fame Table Tennis champion, which the musical does not really touch on but I think is too hysterical to leave out). My personal favorite fact of the story that I did not believe was true at the time was how, on the day that MI-5’s body, named William Martin, washed up on the coast of Huelva Spain, an American pilot, named Willie Watkins, crash landed near the same spot as the body was left, lived, and even (according to the creative team behind the play) was present at the autopsy of William Martin.
In regard to the British operation being a form of theatre, I would have to say that, while many elements of it are definitely theatrical in nature, with the deceptions and the inherently insane, dollar store novel-esque plan that they come up with, the operation itself would not be theatre. In my eyes, there are three types of deception: theatrical, magical, and indistinguishable. Theatrical deception is when performers present the audience with a lie and are able to be fully aware of the lie and how it is being done. Magical deception is when performers present the audience with a lie, knows they are being lied to, but is unable to discern how it is happening. Indistinguishable deception is when performers present the audience with a lie, but are (hopefully) unaware that they are being lied to in the first place. The espionage operation that MI-5 is undertaking in this story is trying to fit into that third category of indistinguishable deception. This is clearly shown in how seriously the generals take the idea of Ewen Montagu leaking information about Mincemeat to his brother, as any word of this plan getting back to the Nazis would invalidate their work and endanger the soldiers in the invasion of Sicily. As the performers (MI-5) do not want their audience (the Nazis) aware they are being lied to, I would say that their operation, while very theatrical, is not theatre. Overall, Operation Mincemeat does a fantastic job at presenting an entertaining and comedic musical while keeping itself rooted in historical accuracy.