For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Charlie returns to Paris and gets ready to do (verbal) battle with Marion for his daughter. He travels through Paris and sees it with new, sober eyes. He staves off Duncan and Lorraine.
Act II
Charlie tries to get to know his daughter again. He battles with his sister-in-law, Marion, and wins. He's still staving off Duncan and Lorraine, although it's clear that the duo wants to see him and relive the good old days.
Act III
Duncan and Lorraine show up at the Peters' home and ruin Charlie's plan; he returns to the Ritz bar to have his daily drink, and wonders how long he'll have to pay for the mistakes of his past.