The season ends with a whimper of existential dread. Don takes his children to see Megan act in a terrible community theatre production. He realizes she is a mediocre actress. After the show, he hallucinates the ghost of his half-brother, Adam (who hanged himself in Season 1). Adam warns Don that he is a phantom, haunting his own life.
Business-wise, Season 5 is about the pursuit of the Jaguar automobile account. The pursuit takes the partners to London, to racing tracks, and finally to a seedy hotel room. To land Jaguar, the partners must make a Faustian bargain with the sleazy Herb Rennet (played by Mark Moses), a dealership owner who will only vote for SCDP if he gets to sleep with Joan Holloway. Mad Men - Season 5
The aftermath is haunting. Joan becomes a partner, but she loses a part of her soul. Don looks at her with disgust in the elevator. The scene where Joan returns home, takes off her dress, and silently weeps in her bathroom is a wordless masterpiece of trauma. The firm gets Jaguar, but the cost is the last shred of their moral high ground. The season ends with a whimper of existential dread