Olivia is awakened from a dream of making love with Peter - by a phone call from Peter. He and Walter have had a breakthrough with the Machine, but they need her help.
At the lab, Olivia's still a little weirded-out by the dream but focuses on Walter and Peter's demonstration of the biomechanical interface that Peter will use to control the Machine. They want to test it, and they need her to ask Broyles for authorization. She'll do what she can.
On a dark highway in southern Vermont, several vehicles abruptly lose power. The motorists watch an airliner crash nearby. Suddenly, all the vehicles turn back on.
Olivia and Astrid meet up with Broyles, who says a disturbance that originated around this highway caused the plane's electrical and hydraulic systems to fail. Walter and Peter arrive, and Olivia observes that Peter seems to be drawing Walter out of his shell.
The vehicles are magnetized. Walter says the electromagnetic force here momentarily increased dramatically, causing the electrical systems in the vehicles, and the plane, to fail. It was strong enough to leave a residual magnetic force. As Broyles and Astrid head to the crash site to get Walter some samples, Walter tells Olivia and Peter that he saw a sign down the road for delicious homemade rhubarb pie. Who's hungry?
Westfield, population 584, seems deserted, but the diner's open. Walter orders while Peter uses the restroom and Olivia, who can't get a cell signal, tries a nearby pay phone to check with Astrid. Things quickly turn weird when the counterman swings abruptly between friendly and murderous, attacking Walter with a knife! Olivia, who found the pay phone dead, returns and shoots him. Walter notices that the man's eyes have double irises.
Peter had discovered a badly injured man, Cliff Hayes, in the storeroom. He needs a blood transfusion immediately. They carry him to the SUV and drive out of town . . .
. . . and right back into town, past the same welcome sign. Cliff says they can't leave. Walter says he's right. They're trapped here.
At the sheriff's department, Walter tends to Cliff, who says that everyone in town went crazy three nights ago. People were delusional, like they had memories from different lives. Most got sick immediately, but others took hours or a day. It started with confusion and tremors, then got worse.
The handful who were unaffected gathered at the high school, including Cliff's wife and daughter. He'd gone to the diner to get food. As they arm themselves for the trip to the school, Olivia mentions a nearby military base. Maybe this is an experiment gone wrong, like in Adina, when the whole population was disfigured? Walter's never heard of it. Peter investigated a case in Adina, with his Walter and his Olivia. Right, says Olivia, confused. She must have read it in Peter's debrief.
En route to the school, Olivia's right arm shakes violently, and she feels dizzy, then talks gibberish for a second. Is it happening to her too?
Cliff's wife, Angela, and daughter, Lacey, run to greet him. One survivor, Teresa, is now sick. Walter examines her, finding she has two sets of teeth. She alternately says her husband's alive and has been dead for seven years. Peter encourages Olivia to have Walter test her blood too. Walter does, saying it's probably nothing. And if not, they'll deal with it together.
Olivia and Peter chat about Walter, whom Peter says is a lot like his Walter. She impulsively asks what the other Olivia is like. He reminisces, saying she gave him a place to call home, and fondly recalls their Friday-night routine of watching cheesy horror films and ordering from Damiano's.
Cliff tells Olivia that, last fall, he turned down a teaching job in Philadelphia. If he'd gone, his family would be safe now.
Teresa manages to escape and kill herself. Walter finds 92 chromosomes in her DNA - twice that of any normal human. He and Bell once combined a tiny portion of this universe with the alternate universe, in a goldfish bowl. The effect on the physics in the overlapping areas was catastrophic. Somehow, this Westfield is merging with the Other Side's Westfield, trapping them in the overlap. Likewise, the townspeople are merging with their doppelgangers. The effects can't be reversed . . . but Olivia's blood is fine.
The survivors, like the Fringe team, don't presently have doppelgangers in the other Westfield. Walter wonders what energy source could merge an entire town, and Peter says what about amphilicite? But why would David Robert Jones want to do this?
The town begins vibrating intensely. Walter and Peter finally figure out that, like the eye of a hurricane, the center of the merger will be a refuge, where the opposite forces cancel out. They get to safety after a harrowing journey, and the town disappears around them.
Later, Broyles tells Olivia they found several unusual devices containing traces of amphilicite around the town's perimeter. The amount recovered is only a fraction of Jones's stockpile. Cliff tells Olivia that his family has lost everything, but they still have each other. And they'll face the future together.
Olivia's relaxing with a glass of wine at her apartment. Peter knocks. He hopes she doesn't mind; he thought he'd see how she's feeling. No, why would she? He asks what smells so fantastic? She laughs and says it's Friday, I ordered Damiano's . . . and kisses him. Surprised, he backs up. She shakes her head slightly as he stares at her in shock.