What can I say without spoilers? That, like Alien, you feel you must walk into the screen like Alice through the Looking Glass and fall into the almost tangible space just out there. Some episodes have seemed like dreams, but this is more like a memory. Incredible realisation of an entire universe. This may partly be because this is the universe I lived in, in my earliest memories - I was age three at the time this episode was set.
Some episodes work as big stories, with one big message, but this one was a series of lovely scenes and performances. It didn't have the big bang of a massive creative explosion - probably coming up in later episodes - but the small scenes and little movements of the men and women - fantastic, funny, sad, and with care for minute particulars.
Review by Alan Sepinwall here.
PS - Directed by Roger Sterling himself! Outstanding work.
PPS - actually I do know what the big message of this episode was: 'You can't use the past to predict the future'; even a powerful past like this. It's basically a hopeful message.