Peter Bishop (Joshua Jackson) has been trying to understand and use the Machine to return to his original timeline, with Walter (John Noble) having become more open and willing to explore the nature and mechanics of the device.
As Cliff recovers he explains that three days ago the population started suffering from a delusional outbreak, acting as if they had lived a second life and killing friends and loved ones as if they didn't know them.
A survivor kills herself after coming down with symptoms of the malaise and Walter realizes that those affected have twice the normal count of human DNA and have a higher rate of mitosis which is creating duplicate body organs.
Due to the inability to leave town, Walter surmises that someone has merged the two realities into a single location, an experiment that he and William Bell had performed on a much smaller scale years ago when studying the parallel universe.
Suddenly, the town is struck by seismic effects, and from the school's roof, the Fringe team and the survivors witness trees and buildings shimmer out of existence.
The remainder of Fringe division is able to reach the survivors and help them; Olivia learns that they found several strange devices equipped with amphilicite planted around the perimeter of Westfield and believes them to have been set by David Robert Jones for some nefarious purpose.
In a June 2012 interview with Entertainment Weekly, actor John Noble called the episode one of his favorites of the series because "it was the first time that really the old team had got back together again — Walter and Olivia (Anna Torv) and Peter (Joshua Jackson).
Reviewers noted that the episode appeared to borrow several elements of other science-fiction and drama shows, including The Twilight Zone, Close Encounters of the Third Kind, The X-Files, The Prisoner and The Walking Dead.