It's not exactly a stretch to say that the episode is structured in three parts to show Don's past with his first wife demonstrating why she remains stuck in an untenable child-like mode, his present with his second wife and why her unstable situation is untenable for him, and his hoped-for future at his past employer Sterling Coo.
After a long span of increasingly airless personal drama, followed by last week's intrusion of a major historical tragedy, the show got back to its advertising roots with a vengeance. In fact, the show may have re-booted itself, as it did at the end of Season 3. For once again, the old Sterling Cooper etc. is no more.