Peggy quits SCDP – Episode 11, 'The Other Woman'
There's only so much lack of appreciation a person can take, and even Peggy Olson has a breaking point. After being overlooked yet again in favor of talented new kid Michael Ginsberg, the SCDP copywriter gives herself an upgrade – in the form of a copy chief position at rival firm CGC. Don takes her decision as a personal affront, and tells her not to bother giving two weeks' notice. But when she extends her hand to say good-bye, fighting back tears, Don realizes he's lost his best collaborator. He presses his lips to her knuckles and lingers there for several seconds, unwilling to let her go.