The painting from Midge is titled "Afterimage" and that sums up this episode, as the episode examines what we saw before, how that can change, and whether our afterimage of an event forever prejudices us from re-evaluating someone. In otherwords, do you really have one chance to make a first impression?
Sally has a storyline with Glen from previous seasons, and they have a "divorced kid" bond going on. He gives her advice and she hangs out with him. However, Betty has an afterimage of Glen as the creepy kid who liked her and caused her all kinds of problems in the neighborhood. She catches Glen with her daughter and she is so scandalized she finally wants to move out of the house.
Midge also returns as Don's former mistress, and he has an afterimage of her as a bohemian artist and had good feeling about her, even though her ended the affair. His afterimage is shattered though, as he learns she is on heroin and is married to a loser who offers to whore her out. He takes pity on her and gives her some money for a painting, and her painting leads him to a business decision.
Speaking of which, everyone has an image that SCDP is dying, which is true. The running joke is all new business they try to get say "six months". Everyone knows Lucky Strike was there business and they reek of desperation.
After Don runs into Midge and sees how he is repulsed by her desperation, he throws a bomb with a full page ad in the Times saying they are through with tobacco. This so upsets Bert he quits, as the ad is basically crapping on his entire reputation.
However, this sets up the final as Don lets it be known they are ot desparate and the partners put their own funds into the company to keep it afloat. They are looking for a wholesome company to save them. Finally, Don repays Pete for his falling on his sword over the aviation account earlier, by paying his partner dues. What does the final hold?