No need to recap the plot or echo consensus points.
From the minute he sees her slinking down the stairs in that spangled ankle bracelet, he's hooked. Walter Neff's already boarded that long, lonely trolley down the one-way track. Yes indeed, sultry Phyllis appears to be just the ticket he's been looking for. Great noir classic. All in all, Neff should have paid attention to that other member of the oddball triangle. Old man Keyes may be a born cynic, but despite himself, he's a father figure looking for a son to take his place, and warning Neff about the "Margie's" of the world. What he doesn't know is that this "Margie" definitely doesn't drink out of a bottle. What's more, Neff's already chosen to ride with the flashy crowd, get out of that dumpy apartment, and get into Phyllis's vicious little insurance swindle. As Keyes tellingly remarks, "You're not smarter than the rest, Walter, just a little taller." I like to think that Walter finally realizes his folly in that brilliant final scene, even if it is too late. Still, the film's cynical veneer is misleading. Because beneath all the deceits, betrayals, and ironies, lies a lighted match and one of the odder father-son relationships in Hollywood annals.