Spoilers for the entire series.
The central secret: \"Don Draper\" is a stolen identity. Don was born Dick Whitman, a poor, unwanted child. During the Korean War he took the name of his dead commanding officer. The early seasons slowly reveal this. His half-brother's reappearance, Pete's attempt to blackmail him, and eventually Betty's discovery of his hidden past all threaten the life he built.
Season by season: In Season 1 Peggy rises to copywriter and, in the finale, gives birth to Pete's child, which she gives up. Season 2 deepens the crisis in the Draper marriage. Season 3 ends with the Drapers' marriage collapsing and the founding of Sterling Cooper Draper Pryce in \"Shut the Door. Have a Seat.\" Season 4 shows Don divorced and drinking heavily. \"The Suitcase\" confirms his bond with Peggy, and he impulsively proposes to his secretary Megan. In Season 5 Joan becomes a partner at a terrible personal cost, Lane Pryce takes his own life after his embezzlement is discovered, and Peggy leaves for a rival agency. In Season 6 Don's self-destruction peaks: he reveals his real childhood during a Hershey's pitch and is pushed out of the firm. Season 7 brings the sale of the agency to McCann Erickson and its absorption. Joan leaves to start her own business, Betty is diagnosed with terminal lung cancer, and Don drives west.
The finale, \"Person to Person\": Peggy commits to her career and finds love with Stan. Pete begins again with his family. Roger settles down. Don breaks down at a California retreat, embraces a stranger whose sense of invisibility mirrors his own, and meditates. The show then cuts to the 1971 \"I'd Like to Buy the World a Coke\" ad, and Weiner confirmed Don made it.
Hidden meaning: Seen as a whole, the series was always building to this merger of the spiritual and the commercial. The falling man of the credits never actually hits the ground, and neither does Don. He keeps landing on his feet and reinventing himself one more time.