What is the main conflict in Double Indemnity by James M. Cain?

In a city where insurance salesmen are close to the bottom of the social pecking order, Walter Huff rates as "average." Still, he is savvy enough to get through the door and get the prospect talking, except that when he makes a call to a certain Mr. Nirdlinger about renewing his auto policy, he ends up talking to his client's wife, Phyllis. Phyllis is attractive, but she looks a little washed out. She seems to have some interest in an auto policy for her husband, but she tells Walter she'll call him and let him know when he can come back.