Whether or not this Leviathan plot pans out in the end, I have to say that I enjoy seeing the Winchester's looking into cases that don't involve the end of the world. It's been awhile since there was a plot that didn't somehow relate to the myth-arc. There should always be at least somestand-alone plots.
This episode sort of connected to other stuff that the show has been doing the last few episodes, but at the same time, it was a fun and semi-dark look into Dean's tendency to internalize all of his guilt. And his guilt is pretty high after killing Sam's old friend Amy. I didn't think that the show would pursue this plot beyond last week's episode, but it's making for an interesting plot.
The whole idea of Osiris is that he puts guilty feeling people on trial for misdeeds they did in the past. He decides whether or not the person is killed based on how much guilt the individual feels. If they feel guilty, they're killed by the spirits of the people they screwed over. Dean ends up on Osiris' chopping block, leaving Sam and Bobby to figure out how to free him.
The format of the episode allows for the return of a familiar face: Jo. Jo died with her mother back in Season 5, but Osiris claims that Dean's guilt over her death makes him eligible to be put to death. What follows is an odd thriller/courtroom case where Sam acts as Dean's lawyer in defending him from Osiris. There was a funny line where Sam says he learned some of his law moves on The Good Wife.
The episode suffers most from the same old stuff: Dean and Sam don't trust each other. Yes, we get it. Yet, for some reason, the writers seem intent on driving it into our skulls. Also, it doesn't help that the show has Sam and Dean lying and hiding things all the time. The show needs to just have Sam and Dean be open with one another so the show can stop falling into the same old trap. However, the show was on point tonight, focusing on a stand-alone case that was enjoyable enough.