Cain's characters live in a world where a maleficent fate sooner or later overtakes them. There are no heroes; there are only ugly people. The protagonists in his stories are all ugly; the really unsettling thing is that you can usually find something of yourself in them. Walter Huff (he's renamed Walter Neff in the movie, for some reason) is an insurance agent who teams up with a woman to knock off her husband and collect on his accident policy. He's not a sympathetic character. We watch Huff go into a long slide, and become a pawn in the hands of people that he doesn't even know before the end - maleficent fate.
The movie, while very good, does not tell quite the same story as the book. Although Cain wrote the novel, the screenplay was written by Raymond Chandler, with an additional credit to Billy Wilder, who directed the movie. Chandler managed to turn Walter into a sort of Philip Marlowe gone wrong, right down to the snappy internal dialogue that was Chandler's trademark. Chandler also added an element of morality into the tale that it didn't have in the book. And why does Walter kill Phyllis in the movie? I don't think I'm giving anything away by saying that this doesn't happen in the book.
If you're hooked on hardboiled fiction you should read this book. And although a lot of years have passed, maybe you'll appreciate why Cain was considered such a racy writer in his day, too.
POSTMAN's drifter is now a cocky insurance salesman (Walter Huff) who thinks he can both beat the odds and get the girl (Phyllis Nirdlinger), and -- why not? -- her daughter Lola as well. If you know anything about Greek tragedy, you can bet that the hubris mechanism is ready to spring into action with jaws agape.
James M Cain writes a tight novella that can easily be consumed in a single sitting. It's just that you feel you've been watching cockroaches mate from a great height. Few of Cain's novels show the least sign of sentiment, let alone liking, towards their characters. Raymond Chandler and Billy Wilder's script for the film is actually far superior because the character of Keyes is developed into a moral center around which the story unrolls. (It also helps that Cain's INDEMNITY has a really gonzo ending.)
Nonetheless, Cain is what he is -- and his stories are always worth reading. But do see the Billy Wilder movie version!
Yet 'Double Indemnity' is a fine read. Not on par with the author's best ('Mildred Pierce', 'The Postman Always Rings Twice') but still among the better in the genre.