Olivia and William Bell rescue Walter from the hospital. Then Olivia meets her double, who is a dangerous woman, trying to find Peter; however they fight but Olivia subdues her. Olivia poses of her double to lure Charlie and convinces Peter to return. Meanwhile Walter and William bring a piece of equipment from Walter's laboratory in Harvard to the theater to let them return to their universe. However, the FBI and the Fringe Division also arrive and there is a shootout. Will Olivia, Walter, Peter and William Bell return to their universe?
Sacrifices will be made and both universes may never be the same again after Walter and Olivia visit the "other side."
- Peter (Joshua Jackson) takes a scenic Zeppelin ride over the Other side's New York City. The automated tour tells him about Fringe events which occurred around town, including a wormhole prompting Madison Square Garden to be quarantined with 10,000 people trapped inside. They have since been declared dead.
Liberty Island is the headquarters of the Department of Defense.
Walternate (John Noble), the Secretary of Defense, gets a report that he himself was admitted to a hospital.
Other Broyles (Lance Reddick) wakes Other Olivia (Anna Torv) up.
Peter visits his dad's office, it's beyond strange. "I've seen strange, this is something else," he says.
Olivia and William Bell (Leonard Nimoy) check a hospital for Walter (John Noble) and get there two steps ahead of Other Olivia and Charlie Francis (Kirk Acevedo). Olivia (Anna Torv) goes to search for him as Charlie has the hospital purge all Walter's records. Bell waylays the Other Fringies with tales of being old pals with Walternate as Olivia finds Walter. He's jazzed about their drugs and miraculous healing techniques. Olivia gets him outside to William's waiting car.
Other Olivia checks the hospital's security footage and is startled to see herself escorting Walter down the hall.
Back in Walternate's office, Peter asks about the red areas on the map of the country, including one on almost all of Boston. Quarantine areas, because of what "that man," Walter did.
When the two worlds interacted, it caused "soft spots" on the other side, Walternate says, but it was worse here.
He's hoping Peter can help. His side is ahead in engineering and science. He didn't recognize the blue prints.
His engineers say the problem is with the power source, the waffle iron-looking thing we saw Walternate with last week.
Other Olivia drops by. Peter can't help staring. Walternate talks to her privately. He tells her their doubles can't be trusted and will lie. He has a favor to ask.
Cut to Walter, Olivia and William eating KFC, which tastes the same on both sides. Walter points out he's not sure they can get back without the other Cortexaphan kids. Williams says Olivia should be able to open the door, but Walter will have to build a "doorstop" to get it the rest of the way.
Olivia's going to look for Peter. They make plans to meet back at the opera house where they crossed over in 12 hours.
Other Olivia takes Peter to a hotel. She tells him when people find out he's back, he's going to be famous.
People know his story. She asks about Olivia. Peter says she's always trying to make up for something and seems haunted.
William and Walter drive through a charred wasteland outside the city. Looking at it, Walter asks if he's responsible for it. Bell tells him he is.
They arrive at Other Harvard, which is smack up against a quarantine zone. Bell tells him Walternate tried for years to figure out how to cross over, but failed. He thinks the equipment they need will be there. He explains Other William Bell died in a car crash years ago so he and Walternate never worked together.
Walter accuses him of pilfering the other world's technology for profit, but Bell reminds him he only came over to try to stop the events Walter started by coming and stealing Peter. Bell says he had no choice but to work for Walternate, coming up with things like the shape shifters. Walter compares that to his 17 years in the mental institution then rails at Bell for cutting out pieces of his brain and stealing his memories.
The conversation is stopped when they find the equipment they're looking for.
Other Olivia comes home and gets dinner from the fridge but turns around to see Our Olivia pointing a gun at her.
In the lab, Walter worries about Olivia being strong enough to keep the door to the other side open on her own. "Leave that to me," Bell says.
Back with the Olivias, Ours says she needs to talk to Peter. She sees a picture of their mother and asks about her. Other Olivia says her sister died during childbirth a few years ago. They both have Ellas. "Please, you've gotta trust me," Our Olivia says. "I'm you."
Back in the lab, Walter asks William why he removed his memories. Williams says only he had his reasons. They're done with their project.
Back with the Olivias, Ours learns they aren't completely alike. Ours keeps her back-up gun in her purse, Other keeps hers in her jacket, which she proves when she draws on Ours.
Other thinks things are personal for Ours, and asks if she and Peter are a couple.
Other puts in earpiece to call Fringe and Ours takes the opportunity to lunge at her. Soon, the Olivias are engaged in a meta-chick fight against themselves. Other is winning and has Ours in a chokehold when Ours manages to grab for a lamp and bash Other in the head.
Ours does a quick dye job in the bathroom and leaves the apartment. She runs into Charlie in the hall. He says they're supposed to check out a power surge at Walternate's old lab, but she says they're supposed to move Peter first.
Peter works on the power source, trying to make it work. He narrates his thoughts, recording. He wonders if it might be symbiotic, needing to work with something organic. He passes his hand over the opened up waffle iron thingy and round nodes stick to his hand.
Other Olivia wakes up in her apartment.
Peter is getting farther with the energy device, realizing it's designed to work with one person: Him.
Olivia and Charlie arrive at Peter's hotel. Olivia starts telling him she has a message from his father: his friends are there and they've come to keep him safe. She knocks Charlie out.
He deduces it's the Olivia he knows.
Peter tells Olivia what Walternate told him, lying about fixing the other universe, when really it was about destroying theirs. He asks how long she knew about where he was really from and she tells him a few weeks. She says he doesn't belong here, and he agrees, but he thinks he doesn't belong there either.
She tells him she's thought of 100 reasons why he should come back but the best reason is his belonging with her. She steps closer. She kisses him.
At Fringe Division, Other Olivia comes in with Walternate. They go on high alert searching for Our Olivia and electronically locate her at the opera house.
At the opera house, Olivia and Peter drive up to find William and Walter outside with their device. Walter is speechless over seeing Peter again.
Sirens quickly approach. Williams tells Peter and Walter to set the device up inside while he and Olivia hold the Fringers off. He reaches into his car for what looks like a gun. It takes out an SUV with a single shot. William hands Olivia phosphorous grenades, prototypes, as Walter powers up his machine inside.
Outside, Other Broyles and Other Olivia get a message from Walternate in their earpieces. They understand.
Outside, there's a huge explosion and everything goes dark for a minute. Olivia picks William up off the street, he was knocked out. Apparently the grenades work.
Inside, Walter thinks they won't have enough power to make the device work but William says his messed up atoms from universe hopping should do the trick.
As the door is opening ,he tells Walter why he took out part of his brain: because Walter asked him to, because of what he was becoming.
The Fringers bust through the door and William raises his hands to the device. Energy flows from him and he slowly begins to dissolve when in a flash, it's all gone and they're back in the opera house on our side.
Back home, Olivia comes home and seems to find her surroundings strange.
In the lab, Astrid (Jasika Nicole) feeds Peter welcome pie. He says good-night to Walter, who asks if he's staying.
"I'm trying to see this your way, Walter. I can't. But, you did cross universes twice to save my life, so that's gotta count for something, right?"
Olivia walks into the store we know serves as a doorway to the other side and asks for the Selectric model 251 typewriter. The man at the counter recognizes the code and gives her the key.
In the back room we see the tattoo on her neck. It's Other Olivia. "Infiltration achieved," she writes. "Awaiting orders." Orders never seen.
Back on the other side, Walternate visits Our Olivia, locked away in a padded room. She pounds on the glass and begs to be let go, but he closes the door, leaving her in complete darkness.