It is May 20, 2026. A badly injured Peter, now age 45, gets treated at Fringe Medical in New York City and is almost instantly healed. Astrid is there, as is the newly minted Agent Ella Dunham, Olivia's grown-up niece. Olivia is the boss now. Ella asks if Peter remembers what he said in the field, about the machine and being from the past? Peter laughs it off.
During a performance at the Brooklyn Opera House, a terrorist named Moreau and two minions set up cylindrical devices. When they detonate, everything goes white. In the aftermath, Astrid reports that they found an electro-light, apparently a dud. It's the first one they've seen. Back at Fringe HQ in Boston, Peter tells Olivia that only one person can figure out how this device functions: Walter.
Peter visits a subdued Walter in prison. Walter has heard about a massive increase in kappa radiation. Peter explains that a wormhole opened in Central Park, and it took months to amber it over. But only a wormhole through time would emit that type of radiation, Walter says. Yes, the carbon levels were consistent with the late Paleozoic era, 250 million years ago. Peter shows Walter the light bomb, explaining that a terrorist group called the End of Dayers uses them on soft spots, trying to accelerate the Biblical end of days.
Peter gets Senator Broyles - whose right eye is now milky and blue - to grant Walter a temporary furlough. Soon, Walter is back in his Harvard lab. He officially welcomes Olivia to the family . . . although the wedding was a while ago. A soldier drops a box of Walter's equipment, and Olivia catches it telekinetically. She can control her ability now.
Moreau asks a shadowy figure what the next target is. The wormhole in Central Park, replies a familiar voice. Walternate! Moreau shows off an even more powerful light bomb, built to Walternate's specifications.
Armed troops guard the lab. Peter arrives, placing some Red Vines on the table, as Walter says that he didn't understand until too late that their worlds were inextricably linked. He says the day we triggered the machine was the day we died. They argue about who's responsible for all this. No matter who's at fault, says Peter, you're my dad. He leaves. Walter sees the Red Vines. He smiles and bites into one, savoring it happily, sadly.
The investigation leads to a campground, where an agent gives Peter a small black case containing a key. He doesn't tell Olivia. Meanwhile, Moreau places light bombs at the amber wall in Central Park. Soon, Olivia is alerted to a security breach at the wormhole.
Peter goes to the cabin at Reiden Lake . . . and comes face-to-face with Walternate, who knew Peter would recognize the key. Walternate says that he came Over Here at the end, to ask for help. And Peter destroyed his own people. Now Walternate's going to destroy Peter's universe. But slowly.
Ella, Olivia, and a Fringe team swarm Central Park. Suddenly, there's a flash of white - and the wormhole is active again.
Peter tells Walternate he came alone to make a personal plea. He's sorry for destroying their world, but Walternate has to stop. You're coming with me now, Father, says Peter. Walternate angrily says that if he was really there, he might have to kill Peter. But he'll start by killing someone Peter loves. Peter lunges at Walternate - but it's a hologram!
In the park, Olivia turns and sees Walternate. He wordlessly raises a gun . . .
. . . and shoots her in the head!
Olivia's funeral is at night, on the beach. Torches burn as Peter eulogizes his wife. Ella, Astrid, Broyles, Walter, and Nina Sharp are among the mourners. Tears flow as Peter lights a raft bearing Olivia's coffin. Afterward, Walter suddenly begs Ella to take him to his lab.
In the morning, Walter tells Peter he finally understands how the pieces of the machine got buried millions of years ago: He himself sent them back through time, using the Central Park wormhole. It's a paradox. Walter can't change what happened, because it's already happened. But Peter can make a different choice WITHIN what happened. Walter simply needs to somehow show Peter the consequences of making the same choice. Then they can cheat the rules of time!
But imagine the repercussions, Peter says. Walter says whatever the cost, it can't be worse than this. Peter asks what he would need to do.
Suddenly, Peter's back in the machine - in 2011. He's been in it for 60 seconds. On the Other Side, Brandon tells Walternate that the soft spots are getting worse. Walternate has Bolivia brought in and shows her that she can turn off the machine. Realizing what's happening, she taunts him instead.
Olivia climbs up to Peter, who greets her happily. The Other Side's lab pops into the room, complete with Walternate, Brandon, and Bolivia. The machine releases Peter, who says that he now understands what the machine does and where it came from. The First People, says Walter. Yes, says Peter, but the First People are us. Walter, most specifically. He tells the two Walters that Doomsday is worse than anything they could imagine - and that their worlds are inextricably linked. He's torn holes in both universes, which lead to this room. "A bridge," he says, "so that we can begin to work together, to fix -"
And Peter simply disappears. No one even reacts. Instead, Walternate and Walter accuse each other of being horrible. Olivia says that maybe it's time we start to fix things.
On Liberty Island, many Observers are gathered. December says that September was right: They don't remember Peter. "How could they," says September. "He never existed. He served his purpose."