The guileless amorality of pre-code Hollywood is always pleasing to me. It's a reminder that the sacred hobby horses of the modern world were often deliberately constructed and recently at that. The Mystery of Mr. X pits the elites of London society against one and other often showing that none of them are nearly as smart as they think themselves to be.
The Mr X of the title is a serial killer targeting policemen on the beat. At the start of the film X claimed his fifth victim. Five policemen dead by the same hand over a brief span of time would rightly send any municipality into a panic. The police commissioner proves incompetent in managing the crisis. While the spree was underway he absconded to a spa retreat in France. Cornered by the press on the trip back he can barely dodge their questions. He certainly can’t put together a plan to either catch X nor keep his officers safe. I’m sure anyone watching this could think of some obvious ways to thwart X, doubling up patrols came to my mind, but I like that the commissioner is clueless. Besides his trips to a spa he lives in a well apportioned mansion with fine dining ware and waitstaff. He probably didn’t come into these possessions solely through civil service.
By chance, one of X’s murders happens alongside the theft of a famed diamond and the police move under the assumption that the thief must also be the serial killer. Though a stretch of logic, the police are fully committed. This comes much to the chagrin of the thief, Nick (Robert Montgomery) who’s now holding a diamond too hot to sell and with two partners close to nervous shock looking to save their own skins as the dragnet heats up.
Nick is no member of the elite, but he dresses to their standard and speaks their language. The spoils of his thievery are intentioned to provide a life of luxury abroad. Instead of laying low with the diamond, Nick decides to help the police find X. He devises what to his mind is a brilliant plan and conspires to enter into the commissioner's graces to deliver it.
The swirl of incompetence, foolishness, and swapping of alibis occupies the greater part of the picture. Eventually X is tracked down and director Edgar Selwyn stages a wonderfully taut action piece to conclude the picture.