As the victims of a highly intelligent serial killer stack up in the Alternate Universe, the Fringe Division "over here" is asked to assist. As the two sides tangle and innocent people remain at risk, the suspect's doppelganger, a professor who teaches Forensic Psychology, is brought into the fold and forces the team to consider the notion of sending civilians to the other side.

Olivia uncovers a dangerous force that may be connected to Walter's past experiments with the drug Cortexiphan. Their investigation forces Walter to leave the lab for the first time in years as he and Olivia head to Massive Dynamic in New York to examine old case files. Walter struggles with life outside the lab as he confronts the demons of his past.