"We've been looking for you." - Unnamed police officer, The Milk & Honey Route
Don Draper is now officially a hobo. Having left his job, moved out of his penthouse apartment, paid off his second ex-wife, and thought his first one (and their kids) no longer needed him around, Don set off on a walkabout that landed him in the heart of what was once the Dust Bowl and ground zero for the privation of the Great Depression that defined his childhood. That he would hand the keys to the one possession he still owned (a two-door Cadillac Coupe de Ville) to a con man who almost made Don's visit to Oklahoma far longer than he would have wanted, was of a piece with Don's comfort around people who are running from something. 
But that is all simply after action reporting. When your dreams are haunted by the fear that you will one be discovered as someone you are not, can you truly live? Well into 1970 and twenty years after his deceit,  that the man born Dick Whitman still has nightmares about having his true identity exposed is its own personal hell. Of course, Don has had close calls before. Whether it was his half-brother Adam showing up unannounced in New York City,  Pete's attempt at blackmailing Don into making him Head of Accounts,  or Don's demand that SCDP drop North American Aviation as a client for fear that a background check will reveal his lie,  like a fugitive who has evaded arrest, Don must always keep an eye out for danger.
In fact, it was not his habitual philandering that caused Betty to demand a divorce, but rather, her discovery of his alternate life - of the manufactured person "Don Draper" was versus the man who grew up dirt poor in "coal country, Pennsylvania, by way of Ohio."  The one person other than Betty that he ever shared his secret with, Dr. Faye Miller, encouraged him to both confront the trauma this dual life was creating for him and to investigate whether he could seek some sort of pardon for his actions.  Don's reaction to these suggestions was to run directly into the arms of a woman he hardly knew and away from the kind and nurturing woman who was trying to help him. So it was understandable that Don Draper had reservations about attending an American Legion fundraiser where other veterans might be present, not just because of Don's sense of inadequacy in their presence,  but the infinitesimal risk that someone who knew his true identity may call him out.
A small river of booze and war stories had to flow, including one shared by another veteran of it-was-him-or-me life and death struggle, before Don could get to the simple truth he had been hiding for so long. "I killed my CO." A simple declarative sentence that was as basic as it could be misleading. To a casual listener, it would suggest cold-blooded murder. That he had waited twenty years to unburden himself felt like both a great relief and an odd way to describe the circumstances that attended his departure from Korea. Even then, he omitted the fact that he stole his Lieutenant's identity, but the deed was nevertheless done. That the veterans shrugged it off with a "you did what you had to do to get home" indifference and a triumphant singing of "Over There" must have been as much a surprise to Don as being awoken hours later by the same crew accusing him of theft.
Had Don been in New York instead of on a rich man's walkabout, he might have learned that his first wife was diagnosed with terminal cancer. Death has an odd way of providing an after-the-fact slant on a person's life. Were we to have known that Betty had only seven years to live when she met Henry  or delivered baby Gene,  the scope of her life would have looked much different. But Sally had it wrong about her mother (a constant theme between these two, who never seemed to accept or understand one another even though they shared a stubborn streak). Betty was not treating her death as a tragedy, but rather with nobility. Betty would not let Sally feel at sea as she did when her own mother passed away. Her detailed explanation of the "arrangements" she wanted were practical, while lacing her farewell letter to her daughter with a humanity Betty was not able to show very often.
On the other hand, the burden placed on young Sally's shoulders could not be greater. The panoply of adult issues presented to her before she is even old enough to drive is truly alarming. While divorce may seem run-of-the-mill in today's day and age, Sally was exposed to it in another time when such action was far less common. Were that not enough, she saw her step-mother's mother fellating one of her father's co-workers,  walked in on her own father having sex with another woman,  and is now being handed the awful news of her mother's impending death.
Betty's cancer diagnosis is another period at the ends of a sentence when it comes to the show itself. As destinies are revealed, all the other options start to fall away. Viewers who want the finality of knowing what happens to the characters they have so assiduously followed for so long are left to ponder how these fates square with their own preferred outcomes. That it is implied Betty's untimely demise may have been due to cigarette smoking is ironic considering the central place that activity has played since the show's inception, but the idea that a person would die from complications associated with that activity are not particularly revelatory to us in 2015.
Pete seems to confirm that Joan has ridden off into whatever sunset awaits her and her ascot wearing suitor Richard as he prowls the halls of McCann attempting to fend off the professional advances of Duck Phillips. But the idea that this "sour little boy"  could somehow find a redemptive streak as a family man in Wichita, Kansas would come as a massive surprise to anyone who has followed the litany of bad behavior and petulance that has been Pete's calling card since the day we met him. 
Of course, the advertising profession is built on spinning a story that pulls at heart strings, whether it is true love or nostalgia for one's past, so it is unsurprising that Pete wants to place rose-colored glasses on his life with Trudy while conveniently forgetting everything from the fact that he swapped a chip 'n' dip wedding present for a rifle,  told her father that he did not love the man's daughter,  and had a pied-à-terre cum shag pad in New York. These indiscretions, both great and small, are easy to forget when you are approaching middle age and uncertain of your future, but it is particularly curious to see a man who wanted to be in New York City if nuclear holocaust was upon us  willing to pull up roots for the middle-of-nowhere Kansas. It is left to Trudy to at first splash cold water on the idea of their reunion, after all, her memory is far longer and the failings in their marriage for more his responsibility than hers, but ultimately, she buys into his tale of a second chance, after all, they say second marriages are the triumph of hope over experience.
And while the Campbells have been hinting at reconciliation and a second chance for some time now, the question that remains unanswered is what Don will do with his third act. Will Betty's cancer diagnosis lead him back to New York where he will take responsibility for the three children she is going to leave behind? Continue his hobo ways, a lone Sears shopping bag containing all his world possessions until he reaches Long Beach, kicking himself that he sold the Craftsman cottage Anna lived in when she passed away?  Perhaps there was a hint in Don's treatment of Andy, the grifter who tried to frame Don for the $500 theft. Instead of punching Andy out, Don admonishes the young man to get his life straight, to not have his first foot in the adult world a false one that he would forever be running from. Don likes collecting wayward souls and trying to direct them to a future that does not include the mistakes he has made, but he seems incapable of following his own advice. It will be fascinating to see what he does when confronted with the need to be a father to children he has disappointed over and over, to "do the work" as Freddy admonished him to do  or take the lure of a modern day riding of the rails to reinvent himself once again, and in yet another guise.
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1. In Season 3, Don picks up a couple who claim to be eloping to Niagra Falls. It turns out they are grifters who drug and rob him. Seven Twenty-Three, Season 3, Episode 7. Don also gives money to Suzanne Farrell's brother, an epileptic, and agrees to not drop him off at the VA Hospital where Suzanne arranged for him to get a job. The Color Blue, Season 3, Episode 10. And let us not get into the backstory of Diana, the Sad Waitress. See generally, Severance, Season 7, Episode 8, New Business, Season 7, Episode 9.
2. Betty's letter to Sally is dated October 3, 1970. Don's reference to fighting he saw around the Yalu River in Korea would peg his service as sometime during 1950.
3. Apartment 5G, Season 1, Episode 5.
4. The Wheel, Season 1, Episode 13.
5. Hands and Knees, Season 4, Episode 10.
6. Don's characterization of his youth is found in My Old Kentucky Home, Season 3, Episode 3. Betty's divorce demand occurs in The Gypsy and the Hobo, Season 3, Episode 11.
7. See, fn. 5.
8. In Maidenform, Don and Betty attend a country club event on the 4th of July where veterans are asked to stand and be applauded. Don is reluctant to do so and looks forlorn as young Sally claps furiously for him. Maidenform, Season 2, Episode 6.
9. My Old Kentucky Home, supra.
10. The Fog, Season 3, Episode 5.
11. At The Codfish Ball, Season 5, Episode 7.
12. Favors, Season 6, Episode 11.
14. While Pete's transgressions could take up their own blog post, if not a small book, a few of his "greatest hits" suffice. He affirmatively told Don he wanted Don's job in the show's very first episode. Smoke Gets In Your Eyes, Season 1, Episode 1, he sexually assaulted his neighbor's au pair, Souvenir, Season 3, Episode 8, told his ex-wife that he saw her father in a whore house, For Immediate Release, Season 6, Episode 6, slept with one of the Cos Cob housewives, The Collaborators, Season 6, Episode 3, and floated the idea of having Joan sleep with Herb Rennet to secure Jaguar, The Other Woman, Season 5, Episode 11. He also found a way to make becoming a millionaire sound like a struggle. Severance, supra.
15. Red In The Face, Season 1, Episode 7.
16. The Mountain King, Season 2, Episode 12.
17. Meditations In An Emergency, Season 2, Episode 13.
18. Tomorrowland, Season 4, Episode 13.
19. The Monolith, Season 7, Episode 4.