Air Date: 5/13/10
"Swan Song" should've been the end, if you couldn't tell from the title. By The CW's impossibly low ratings standards, Supernatural is a hit, but back when the network had grander ambitions, a Supernatural renewal wasn't always a sure bet. So when the series defied the odds long enough to actually see its end-game through—the monster-hunting Winchester brothers battling the Devil himself—everyone assumed it would achieve the cult series pipe dream and bow out on its own terms.
Alas, The CW came to rely on the show's fan base, so instead of being a great series finale, "Swan Song" is merely another season ender. A pity, since it would've sent the series out on a high note; the following seasons have unwisely tried to match the stakes of a Hell-on-Earth apocalypse, with uneven results.
Anyway, the episode finds the Winchesters plotting to thwart both Lucifer (Mark Pellegrino) and the bureaucrat-like angels who want to see an apocalyptic battle between Heaven and Hell play out. In the Supernatural universe, angels and demons can only inhabit this dimensional plane through human vessels. An entity like the Devil can't be hosted by just any scrub, and as fate would have it, Sam Winchester (Jared Padalecki) is his one true man-suit.
Desperate and without any better ideas, Sam has the notion that if he lets Lucifer possess him (demons can enter a person at will but angels have to be granted permission, even fallen ones) he can stay in the driver's seat as it were long enough to let his brother Dean (Jensen Ackles) re-open the gates of Hell, and sacrifice himself by jumping in. Of course, Sam fails spectacularly at mental tug-of-war, which is to be expected when the opponent is Satan.
As the Devil-as-Sam plots his apocalypse, Dean plans to appeal to him one last time. Meanwhile, the history of the show's de facto third character, the Winchesters' 1967 Chevy Impala, is narrated amidst the doom and gloom. It sounds corny, but the end result is a genuinely affecting climax that is somber and thus very fitting. It would've been a hell of a conclusion to five years of sibling strife and ghost-hunting.