Crowley, supposedly out hunting for The First Blade (the only artifact known to be powerful enough to slay Abaddon) has gone AWOL, and the Winchesters discover he has fallen foul of his addiction to human blood.
Unfortunately Abaddon's agents have been working Crowley and she has been alerted to the Winchesters' quest for the holy relic.
The brothers retrieve Crowley, drag him back to their lair and force him to go cold turkey, while questioning him about what he has unearthed on the missing blade.
Their studious legwork leads them, eventually, to Cuthbert Sinclair (Kavan Smith), a rogue member of the Men Of Letters, a master sorcerer concealed from the world amidst his vast collection of magical items and supernatural critters.
However, Cuthbert isn't so much interested in parting with the First Blade as acquiring Dean - and more specifically The Mark Of Cain - to add to his collection.
Blade Runners was a good, old fashioned treasure hunt, a straight-forward story with predictable (but nonetheless enjoyable) twists and turns that allowed Mark Sheppard to have some fun with Crowley.
The episode also featured a brief cameo by a demonic Snooki aka Nicole Polizzi. Anyone still remember her given that her 15 minutes of fame dried up years ago?
This is, of course, one of the problems with this kind of quirky 'stunt casting' (even for comedic cameos) - the subject of the joke is likely long forgotten before the show they're popping up in ever is.
Especially if, like Supernatural, we're still about a year behind the States over here!