Then Hamlet wrote a scene for the players to enact
While Horatio and him would watch to see if Claudius cracked
The play was ca'd The Moosetrap, no the wan that's runnin' noo
Sure enough the king walked oot afore the scene was through
I am guessing this is not the first time someone has had the idea of doing a Whodunnit murder mystery which takes place during a performance of a Whodunnit murder mystery. But the idea of someone being murdered during the first run of Agatha Christie's Mousetrap (on the hundredth performance, in fact) takes the meta-gag to a new level.
If you are one of the sixteen people who doesn't know the Very Surprising Twist Ending to the original play, then the movie doesn't reveal it, quite. There is a kind of twist but the twist is that the twist is not quite the twist you may have been expecting if you were expecting a twist. If you see what I mean.
Eminently dislikable film producer Leo Köpernick (Adrien Brody) has come to London with a view to making a movie of Agatha Christie's play, although the contract stipulates that it can only go into production six months after the play finishes it first run. (DRAMATIC IRONY.) He explains the cliches of the English Whodunnit in the pre-credit sequence, noting that it is always the most unlikely character who turns out to be the murderer, and alway the most dislikable character who gets murdered. Right on cue, he is murdered himself. The film is pretty much constructed out of these kinds of meta-jokes; but they crop up with sufficient frequency and lightness of touch that the film never feels too clever-clever. The screen writer explains that flashbacks are lazy plot device, almost as bad as captions which say "Three Months Earlier" -- and a "Three Months Earlier" caption dutifully appears on the screen. Some of the gags are a little in-your-face ("I think they were all in it together!" says one of the investigating officers) and some are the kind where you chortle loudly to show you got it ("This inspector's a real hound!")
I found the actual Whodunnit only slightly more preposterous than Death on the Nile, and very much funnier. I don't find it easy to keep multiple characters and red herrings in my head, which is why I am not a particular fan of the genre, but the individual characters were funny enough that I didn't particularly mind. The denouement follows the staid rules of an English Whodunnit, but adds the car chases and gunfights which Kopernick thinks a cinema audience demands. In the final seconds, the fourth war comes tumbling down, and one of the characters tells us not to reveal the ending. So I won't. It does indeed turn on a plot-point in the Mousetrap, but not necessarily the one you remember.
What makes the film work as a movie rather than simply a theatrical riff is the central turn between Sam Rockwell (Inspector Stoppard: yes, I know) and Saoirse Ronan (Constable Stalker.) The former is a cynical, world weary, slightly alcoholic old cop who has seen it all before; the latter, an eager young newbie who writes everything down, and leaps to all the wrong conclusions, until, obviously, the very end. It's obviously a terrific cliche but that's part of the joke; and it works so well that one assumes that at this very moment someone is pitching a followup in which Stalker and Stoppard have to investigate a murder on (say) the set of the Thirty Nine Steps.
I didn't know that dear dear Sir Dickie Attenborough created the role of Detective Trotter in the first production of the Mouse Trap itself. Harris Dicksinson acts the actor very successfully. I did enjoy him briefly explaining the nature of theatre to the murderer. "I would be the first to admit that the Mousetrap is not Hamlet" he explains. Well, quite.
Wikipedia informs me that See How They Run is also the title of a 1945 farce, which contained the immortal line "Sergeant, arrest most of these vicars."
NOTE: If the play is taking place in early 1953, then the King has just died but the Queen has not yet been crowned. Missed a trick there, I feel.