As Mo Ryan detailed earlier this morning, last night's excellent episode of 'Mad Men' (Sundays at 10PM ET on AMC) showed viewers the full spectrum of the relationship between Peggy and Don. It's a special one, no doubt, one that's developed in interesting ways over the last four seasons.

At times, they've seemed to be boss and secretary or boss and copywriter, but at other times he seems to be the father figure she never had. Sometimes, though, Don acts like an older brother goading and pushing his little sister to achieve. There have even been fleeting moments of sexual tension between the two, but nothing that would or could ever be explored, especially by Don. He knows too much about her, and besides, in her he sees the younger go-getter version of himself.

Because of the ebb and flow of the night Peggy and Don spent together -- Peggy missing her birthday dinner and breaking up with her boyfriend, the confrontation over Glo-Coat, the Sterling tapes, the drunken fight between Duck and Don, Don finally having the nerve to call and find out about Anna -- it was tough to pick any one scene to highlight. But the final scene of the episode ties the night, and their relationship, together quite nicely.

It's interesting how symmetrical the Peggy-Don relationship really is. Don has secrets that not many people know about. Those that do -- Pete Campbell, for one -- want to use those secrets against him. Others, like Betty, see Don's secrets purely as evil lies and deceit.

Only Anna Draper knew and accepted the real Don, and his motives for wanting to assume a new identity, which is why Don did his best to delay hearing what he knew would be bad news about her, and why he needed Peggy there to help him get through the night.

But, as we know, Peggy has secrets, too. Don's one of the few people who knows about those secrets and can understand why she'd want to keep them that way. Others who know Peggy's secret either want to keep it secret for selfish reasons -- Pete again -- or because they're ashamed, like Peggy's horrible mother.

Only Don can understand that people need to keep secrets in order to build a life for themselves that isn't burdened by what's gone on in the past. Which is also why Don can't tell Peggy about his secret... at least not yet.

The look between them as Don shows a sketch of a Samsonite ad based on the now-famous photo of Muhammad Ali standing over Sonny Liston indicates that, even though Anna is gone, there is at least someone in his life that he'll be able to lean on, someone who he could open up to over time who won't judge and who will continue to love him for who he is, warts and all. The handhold between them is a callback to the pilot, when Don Peggy physically reached out to Peggy Don inappropriately, but now it feels more than right.

The bright sunlight that bathed the scene, the grateful look in Don's eyes as he looks at Peggy, the hopeful music that closed the episode, and Don's desire to keep his door open all signal that, because Peggy is there, things might be OK for Don after all. He may have some more benders to go on, his life my continue to spin out of control for a little while, but at least he knows he won't be going through this alone.