Don arrives home, sees Midge's painting, and desperately needing something on which to take out his frustrations, starts to remove the thing to, presumably, the building's trash room. However, he takes a good look at it and decides to go another way, and we cut to him sitting and staring at the thing, as if trying to find meaning and inspiration in its horse-inspired shapes. Later, sitting at his window desk, he rips out everything he's written in his sobriety journal and starts something different, which he titles "Why I'm Quitting Tobacco." Ooh, I always knew that smoker's cough of his they've taken so many pains to show would pay off! Well, actually, it's only tangentially related, as Don, taking a few liberties with who left whom, writes that his agency recently parted ways with Lucky Strike, and he's relieved, because now he doesn't have to participate in selling a product where the quality of work doesn't matter, and that "never improves, causes illness, and makes people unhappy."
As we cut to him typing the statement and then editing it, the voiceover continues that they serviced the brand because there was a lot of money in it; later, he's swimming when he says that they survived on it. "We knew it wasn't good for us, but we couldn't stop." And there's the Midge inspiration he got from the painting, and I like how the show is non-explicitly making the statement that tobacco and heroin are on similar footing with regard to addictiveness. Later, as he continues, we see that his words have been printed in an ad in The New York Times, which we see Henry, Pete, Roger, and then a group on the elevator, including Danny and Joan, reading. DVO tells us that he realized this was an opportunity for him to be able to sleep at night, and as such, SCDP will no longer be accepting tobacco business. Say what you will about Don's impetuousness or self-involvement -- he knows how to attract an audience. And of course, there are many ways in which this play is tactically brilliant, the first and foremost being that their willingness to turn away huge business runs strongly counter to the stench of desperation they've been giving off. Not that you'd be wrong in pointing out that the tobacco companies' response would probably be "Who's asking?" but that's not really the point.