Cain pops up on the Winchesters' radar when he kidnaps a murderer from the Death Row of a Texas prison. Cas tracks him down to a vast killing field where Cain tells him he's got the hunger again and is killing off all his descendants (about 10 per cent of the human population he estimates).
The Winchesters work out who Cain's next victim is likely to be - a 12-year-old boy - and set a trap, with the boy as bait (much to Sam's horror).
However, of course, their only chance of defeating Cain is with the First Blade back in Dean's possession, and so they call up Crowley to return the weapon to them.
Cain walks into their trap, telling Dean that he'd engineered this confrontation so he could get his hands on the First Blade again.
Things don't look good for Dean...
For what should have been a pivotal episode of the season (Dean's prophesied showdown with Cain), The Executioner's Song was rather muddled.
Cain's killing spree might have had more of an impact if it had been alluded to earlier, and explained a bit better.
Even when Cain appears to have turned the tables on Dean, there's no real sense of jeopardy because ultimately that's not what the episode is about. It's about Dean's ongoing battle to resist the Mark Of Cain, to not slay his friends and family as Cain predicts he will.
Probably the strongest moment of the episode (and I can't believe I'm saying this) is the speech Rowena gives Crowley, when he returns to Hell after the Dean/Cain fight, about him being the Winchesters' "bitch".
Ultimately, The Executioner's Song felt like a "necessary" episode to get certain plot points across that the writers' couldn't massage into other stories.
After a run of strong "monster-of-the-week" episodes (that still moved the season arc along, what there is of it), this was an unfortunate return to the patchy, lackluster stylings that dogged the first half of this season.