This is a tightly-constructed mystery of the pre-Black Mask style, in which the solving of the crime -- here a potential serial killer -- must be tracked down, and the only clear clue is a bit of paper at the scene of the crime with a cypher code.
The movie tries to add psychological drama by turning it from a "Whoodunnit" to a "Howcatchem" a style of mystery familiar to all fans of the old "Columbo" TV movie series, with the added punch that it is told from the viewpoint of the killer -- in this case, Robert Preston, who is an American who is somehow running a code-breaking division for the British government. Motivations are established early, but the whole thing is rendered a bit flat by the lack of details that surround the personnel. The result is a well-told story that is not, alas, particularly gripping.
The movie tries to add psychological drama by turning it from a "Whoodunnit" to a "Howcatchem" a style of mystery familiar to all fans of the old "Columbo" TV movie series, with the added punch that it is told from the viewpoint of the killer -- in this case, Robert Preston, who is an American who is somehow running a code-breaking division for the British government. Motivations are established early, but the whole thing is rendered a bit flat by the lack of details that surround the personnel. The result is a well-told story that is not, alas, particularly gripping.