Gage will give them a million dollars if Diana agrees to spend one night with him. "The night would come and go, but the money could last a lifetime," Gage tells them. "Think about it. A lifetime of security for one night." .
While they adamantly refuse at first, the offer becomes more and more appealing because, if there's one thing David and Diana need, it's financial security. They had been leading a storybook life where they eloped because their parents thought they were too young, supported each other through school, gotten good jobs and started building their dream house. .
This is important because it lends an additional aspect in their decision to accept the proposal. They don't do it because they are greedy -- they do it because they're desperate and don't see any alternative. They make the decision together, vowing that they will just forget about it when it's over and pretend it never happened. Diana said to David, "It's just sex not love". The twist in the film, though, is that the sin does not lie with the rich guy, but rather with this unfocused and immature couple who would do such a thing, naively believing it would get their lives on track. .
The night comes, but afterwards they can't forget about it, and its memory eats at the foundation of their marriage. .
David and Diana's relationship deteriorates as soon as they go home. David finds John Gage's phone number in Diana's purse and questions Diana on what happened on her night alone with Gage. They lose all trust in each other and grow further and further apart. After finding out that Gage bought the land that David and Diana want, Diana goes to meet him to try to get their land back. When David finds out that they have seen each other he goes crazy and decides to leave Diana. Without a husband and desperate for some sort of companionship, Diana begins seeing John Gage. After spending a long period of time with Gage, Diana decides that she would be better off with David.
