Meaning of the title?

The title refers to the radio program that was used by the villain to fake an argument between Serova and her young lover for the operator to listen to so as to provide the police with falsified evidence of the alleged fight. A lot of radio shows at the time were entitled 'The ______ Hour' so the film used 'Fatal' to refer to the murders in question.

just a guess but maybe everything in the movie was in a time sequence and everything was sposed to happen in an hour,, or maybe it's like after the murder the first hour after that is the one where it's fatal,, cause it pretty much decides if you catch the killer or not,, who knows,,
are you going to bark all day little doggie,, or are you going to bite