Dan Duryea is once again a man down on his luck, so he opts for a new profession as a kidnapper. His inexperience shows as he kidnaps the son of some working class people who couldn't afford the ransom anyway, plus he accidentally kills the child. No spoiler here, this all comes out in the first fifteen minutes and just as exposition. On revealing that, he tells his wife who is also in on the plot, that the boy was "lucky to have died clean" - as in free of sin.
How considerate Dan! Otherwise, Dan's a nice guy who hung around in Britain after the war, he's nice to his neighbors, and that's where the tide turns. It seems that one of those neighbors, played by Isa Miranda, caught a glimpse of the kidnapper making a ransom call. She offers to help the police capture the man by making it public that she saw him and then just sitting as bait for the criminal.
She only saw the caller from the back, but that's a minor point as long as the caller doesn't know that.
All of this happens in the first twenty minutes, so don't worry about too much being spoiled. Some of it is only referred to anyway as it happens off-screen or even before the film starts.
From here on, as far as the story goes you're on your own. Unfortunately the director Frank Nesbitt not only telegraphs the ending, he writes it in the sky with gigantic letters by fixing the camera on a key prop that comes into play later.
Otherwise, the performances are tops and while it's obviously done on the cheap, that only enhances the look of the film which isn't exactly set among the upper class anyway.