Late in the show's final season, "Lost Horizon" serves as a transitional episode for our characters. Not only are they moving from Sterling Cooper & Pryce to their parent company's headquarters, but they're also finally figuring out — after years of inner tension and conflict — what they really want.
Frustrated by the overwhelming incompetence and rampant sexism in her new work environment at the McCann headquarters, Joan calls on Jim Hobart to step up, but all he wants is to buy out her shares. As Joan reconsiders her life's trajectory, Peggy, the only copywriter currently without an office at McCann, refuses to relocate until a space is found for her. In the meantime, she clings to the emptied offices of Sterling Cooper & Pryce and bonds with Roger, who's also reluctant to leave the world he knew behind. And, during a meeting, Don decides to quietly slip away and start driving west.
"Lost Horizon" also features one of the show's most iconic images. With a cigarette dangling from her lips, a thick pair of sunglasses disguising her hangover, and an erotic painting gifted to her by Roger tucked underneath her arm, Peggy waltzes through the halls of her new office, ready to claim her new space.
