Castle is faced with two disturbing possibilities on Monday's new episode:

1. A friend of his killed his wife.
2. Beckett has arrested an innocent man.

These options, and Castle's reactions to them, are what drive "The Final Nail." See what we mean via the following sneak peeks, as Castle arrives at the crime scene prior to Beckett, argues with her and then ponders the fate of his pal, played by Jason Wiles.

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.