Don watching Megan’s screen test is a critical moment. Clearly something about it changes his mind and we see the season’s arc-of-change really come into focus after that. Yet most people seem unclear about what happened in that scene and it’s critically important to what comes next:
Don is watching a screen test of his wife. This is a reel of film designed to show an actor’s ability. She’s going through her various poses, looks, etc. Don recognizes those looks - they’re the ones he thought were just for him, the sideways glance, the eyes turned down, the hair moved slowly behind the ear.
Previous seasons have shown that Don really understands when he sees things on film. It’s a recurring theme going back to S1. After watching Megan’s screen test he cringes a little. He’s getting wise to her act. And he sees that it worked on him. It must be pretty good.
- - -
Earlier in the episode, Don has a tooth that is rotting away in his head. He ignores the pain, insisting that it will pass. However, he finally gives in and has it extracted. The dentist explains that the tooth was poisoning him, that he was lucky he didn’t lose his jaw.
In the same way, Megan is rotting away. Don is trying to pretend the problems will pass, and everything will be fine. The truth is, if they stay together, she’s going to rot away and Don’s going to be worse off for it.
While Don couldn’t save the tooth, he still has time to save Megan, and to leave his jaw intact — so to speak.
(Source & discussion: r/MadMen)