The workings of the BSU, after the unwanted attention earlier in the season, are laser-focused on the Atlanta child murders, and we start the episode talking to Wayne Williams (who was eventually found to be THE GUY), but they release him in short order. Bill's family issues drive a wedge between Holden and himself, and some old fashioned detective work takes over.
For anyone who's read Douglas's books or remembers this event, there is still a blanket of dread and despair,hanging over the police, and nobody sets the tone for this better than Fincher. A beautifully sad ending to season 2.