We know that Charlie is revisiting Paris and stopping by his old haunts while he's there. As he talks with the barman, we get hints of his past, but the central conflict has yet to be introduced.
This is about as classic plot as it gets. Charlie wants something; it's not going to be easy to get it. Obstacles stand in his way; Charlie struggles against the obstacles standing in his way.
And leaving him nagging letters at his hotel. The story is complicated by a number of different things. Duncan and Lorraine are the obvious choice, but Charlie's past calls to him in other ways, like in the vague sense of nostalgia we get from his reminiscences. And don't forget about the reader's doubt as to Charlie's "transformation." We don't know if he's totally recovered yet, which complicates our understanding of his character and our instinctive desire to root for him against the odds.
In terms of plot, we've been building towards this moment ever since Charlie left his brother-in-law's address at the hotel. It's clearly the dramatic climax of the story as well; emotions run high as Charlie tries to hide his anger, deal with his anxiety, and placate the horrified Marion.
The decision is not yet final when Charlie leaves the Peters' house, but we have a pretty good idea of what's going to happen. Part of the suspense is that we wonder if Charlie will start drinking again after he loses his daughter.
We thought as much. What is surprising about this ending is that Charlie does not take a second drink. He sticks by his one-drink-per-day rule, despite the fact that he's lost his daughter, the emblem of his new future. Why he doesn't is a tricky question, and one that we address in his "Character Analysis."
But Charlie has to wonder for how much longer this atonement can go on. At the same time, Fitzgerald is asking how much longer he and his generation will have to pay for their own extravagance in the 1920s. There is a grim tone, here, as Fitzgerald anticipates that the worst may be yet to come.