This is a Romeo and Juliet sort of story, but told from the perspective of the parent taking control. The outcome is less deadly, but no less tragic for the people involved. Dolly spent a summer in emotional bliss with a man who was bright, interesting, fun, and not particularly well off financially. At the end of the summer, Dolly’s mother sees them engaged in sex (which she doesn’t know that Dolly initiated, but she can plainly see she is fully participating). Instead of getting the message that this is the man for her daughter, she maneuvers to break up the relationship and get Dolly married to a more suitable man. No consideration for her daughter’s happiness at all, and no sense that she has not business in matters of the heart.
What follows is about what you would expect. Dolly may or may not be in a situation where she has to get married for social reasons, but she is clearly conflicted. It is possible that she will learn to love the man she is marrying, but there is a hint that won’t be the case. She wants to take her low maintenance pet with her to South America and he flatly refuses to let her do so. He seemed life a nice enough guy up to that point, but when push comes to shove, he expects to have his way. So the viewer watches the inevitable unfold.