Having (unexpectedly) teamed-up with Rowena, Sam learns that that the only way she can read the Book Of The Damned - and unearth the magics needed to remove the Mark of Cain from Dean - is with a certain codex taken years ago by the Men Of Letters.
The book turns out to be locked in a magical safe - the Werther - created by renegade Man Of Letters, Cuthbert Sinclair (Kavan Smith), who they butted heads with back in last season's Blade Runners.
The Werther has a powerful ward on it that, unless bypassed, drives anyone who tries to open it, and any in the house at the same time, to kill themselves.
The safe remains in the basement of a known "suicide house", but Sam doesn't realise until he breaks in that Suzie (Brenda Bakke), the daughter of the last family that lived there in the 1970s (and all killed themselves within a week of moving in), is still there... and slightly paranoid.
Dean believes Sam has gone "solo" to punish him for his own rogue tendencies, and follows his younger brother to the house, charming his way in the front door while Sam slips in another way, and accidentally sets off the ward on the safe.
The Werther Project was a classic example of Supernatural bringing its A-game, feeding off its own bountiful heritage - without being so 'inside baseball' that it was impenetrable to casual viewers who might be watching for the first time - while advancing its main season arc and delivering a strong story in its own right.
Loved the fact that to crack the code of The Book Of The Damned (which Dean still doesn't know that Sam didn't destroy the other week) Sam needed to find another magical tome for Rowena, who has agreed to act as Sam's agent on the condition that he then kill her son, Crowley.
And despite my general apathy towards the character of Rowena, I have to grudgingly admit that she works better in this odd partnership of her and Sam, although I still can't overlook the pantomime nature of some of her earlier appearances.
Unfortunately, it's the infrequency of cracking episodes like The Werther Project this season that really draw attention to how uneven Season Ten has been.