A commuter train chugs on its merry way. At the station, a teenager panhandles and pick pockets passing commuters. On the train, lights start flashing and a man (Peter Weller -- you know, RoboCop) suddenly appears out of nowhere.
The panhandler greets him when he gets off and is the first to get on board and see what's become of the passengers - they're all dead.
Walter (John Noble
) writes a letter to Peter (Joshua Jackson
), containing the phrase "unimaginable secret." Peter calls, saying there was a train incident and he'll be home in 15 minutes to pick Walter up.
Walter seals the letter and puts it in his pocket.
At the scene, Peter asks Olivia (Anna Torv
) if she's noticed Walter acting weird all week. He's enveloped in a "sadness" Peter says. Olivia says she hasn't noticed.
On board, Walter looks at the dozen dead bodies, seemingly frozen in place. He checks one of the victim's underwear for telltale release of bodily fluids, but finds none. His first guess is collective heart failure.
Olivia notices all the lights are out in just that car.
Peter finds a victim's cell phone is completely dead. One of the FBI guys finds Walter's letter, fallen on the floor. He scrambles to get it back before Peter notices.
He tells Broyles (Lance Reddick
) his heart failure guess is wrong and he wants to take six or seven bodies to the lab.
They get a still image of the man from the security cameras and wonder how he killed everyone on the train.
At the lab, Peter asks Walter if anything's wrong. He says he's fine.
They find that the victim's cells died much faster than normal. Walter concludes that like the batteries on the train, they were drained of all life.
Broyles and Olivia watch the man's path through the city on security cameras, finding he nonchalantly stopped for breakfast at a cafe after he left the train.
At the cafe, a waitress says he's a regular, Alistair Peck, and he's always doing math.
Olivia and some agents storm Peck's home, where complex formulas line the walls and chalkboards. In come the Bishops.
Walter's jazzed by the sophistication of the calculations.
Peck's an astrophysicist at MIT. Walter says Peck has turned Einstein's theory of relativity on its ear, and he'd need tremendous energy to do whatever he's doing.
Outside, Peck watches the agents carry out his things and walks in the front door. The agents stop him. There are bloody surgical instruments and gauze upstairs. Peck tells Olivia the people from the train are only dead temporarily.
"It is well within my ability to make it so that you are never in possession of the things I require," Peck tells them. (And no, we're not entirely sure what this means.)
Walter notices Peck's bandaged forearms and says he's implanted Farraday Mesh, a shield to create a temporal pocket around his body. Walter marvels at it as Peck starts blurring and fading until he suddenly disappears.
He reappears on the train as it's pulling into the station. The same pickpocket greets him outside. "I'm sorry you have to go through this again," Peck tells him as the boy gets on the train and sees the dead bodies for the first time.
Peter leaves a message for Walter as he finishes his letter. It's the same scene, with Peter saying he's on his way.
At the train station, Peter and Olivia have the same conversation about Walter again. Broyles arrives again, Walter draws the same wrong heart attack conclusions again, Olivia notices the lights are out again.
Olivia talks to the pick pocket, who tells them about Peck saying he's going through it "again."
Walter drops his letter again and gets it back.
Federal Building, Boston
Peter calls Olivia to report Walter's findings about the bodies. This time, Broyles finds Peck through a fingerprint on the scene. He worked at NASA.
They bust down his door again. The Bishops look it over but this time Olivia gets really weird deja vu when they find Peck's MIT credentials.
They find two toothbrushes in the bathroom and wait. Peck doesn't come.
Hours later, Walter's ready to go but asks Olivia if he can ride with her. She tells him Peter knows something's up. He tells her about the letter and how he's sure Peter will react badly. He's waiting for something before he tells him.
Peter finds forms for a machine, to cast parts.
Cut to Peck building something then getting dressed. We see his skin is stretched thin over metal and wires (it's Robocop all over again!).
Olivia and Peter meet with Carol Bryce (Laara Sadiq
), who tells them Peck was obsessed with particle acceleration, wormholes - time travel.
She says Peck's idea were over his bosses heads.
Olivia shows her a picture of Peck with a woman that she found in his apartment. Her name's Arlette (Kristen Ross
), his fiancee.
She shows them a manuscript he sent her to proof, but says it's goobledegook. Olivia has just the person to read it.
Back at the lab, Walter finishes the manuscript and announces he'll understand it in about 20 years. He says Peck might be able to travel through time. Walter explains Einstein's theory someone traveling faster than the speed of light might be able to jump around, but would need a ton of power to do so. They think his time travel is what killed the people on the train.
Walter announces if they're right, they've probably already caught Peck at least once.
Astrid has information on Arlette from her car registration as Walter finds Peck's final chapter is titled "Achieving the Arlette Principle."
They find Arlette was killed in a car accident 10 months ago. They think Peck is trying to save her.
"Grief can drive people to extraordinary lengths," Walter observes.
Walter says that based on the loss of life/energy used for a 12 hour jump on the train that morning, a 10 month jump would require a catastrophic amount of energy use,killing hundreds.
Hours later, Astrid (Jasika Nicole
) finds Arlette's cell phone is still in use, at MIT. They think he might have a lab there.
At MIT, the FBI prepares to swarm. Walters appeals to Olivia to let him talk to Peck, as someone who has gone through what Peck has.
In his lab, Peck inserts metal parts into his chest, which has wires coming out of it. He's sodering and suturing his own skin.
Walter walks in to the lab. Peck knows who he is, and Walter says he knows Peck could jump if he wanted, but asks him to stay.
Peck talks to him, scientist to scientist, saying they both know there are places in the world where 2 + 2 does not equal 4.
Walter tells Peck they know when his next jump will be. Walter warns Peck to stay away from the snipers aiming at the windows and Broyles, listening in, worries about Walter's methods.
Peck thinks the jumps back make the victims undead, but Walter says if he saves Arlette from her car accident, those jump victims will stay dead. Peck tells Walter about fighting with Arlette on the day she died, and about having his time travel epiphany on the same day.
He promises Walter he'll jump to the empty field, and only plant life will die.
Walter tells Peck he knows why he hasn't jumped back to save Arlette yet, because he doesn't know how to go that far. Walter stands up and removes his microphone so no one outside can hear them. He tells Peck about an error he's found in his calculations.
Broyles calls for a team to go in.
Walter tells Peck he won't be able to live with the consequences, even if no one dies. He tells him how he stole Peter from another universe. He thinks what he did betrayed God and everything that's happened to him since is punishment. He asked God for a sign that he's forgiven, a white tulip. Peck points out they don't grow this time of year.
Walter knows, but He's God, he can make it happen. And Walter thinks if God can forgive him, maybe Peter can, too.
"Walter, God is science," Peck says as he cites scientific achievements.
The FBI teams run down the hall to storm the lab.
Walter cautions Peck again there will be consequences, and he won't be able to look at Arlette the same. He says he's a cautionary tale. Peck looks like he's considering what Walter is asking when the FBI busts down the door. Walter runs to them to stop them but Peck starts fading and shimmering, preparing to jump.
A police officer outside Peck's apartment radios for back up to help with the dead bodies lying in the street. Inside, Peck calculates frantically.
Back in Walter's lab, Astrid is finding Arlette's cell phone is still active again when Broyles calls to say Peck is back and six people are dead, including two police officers. They have to take him down.
They arrive at his apartment. Inside, Peck gets ready, but first he writes a note, then another to Carol Bryce at MIT. Peck ducks just in time to avoid the sniper fire. Inside, he prepares to jump.
He arrives in the field he told Walter about, a circle of dead grass all around him. He races down the street to Arlette as she gets in her car. He climbs in the passenger side and takes her hand, saying he loves her.
Then a truck barrels down the street and crashes into them.
At MIT, Carol Bryce finds the letter Peck wrote in a file. Inside, there's another letter addressed to Walter, with a note that it should be delivered on a specific date, today.
A colleague drops by and tells Carol to open it, it's been almost a year since Alistair died. She doesn't.
Walter finishes writing his letter to Peter. This time Peter doesn't call with news of the train accident. Walter looks at a happy picture of him and Peter and takes his letter and tosses it in the fire.
Peter arrives, with a present for Walter, his turn table, to cheer him up. Walter says a decision was weighing on him, but he's fine. He watches the letter burn.
The mail arrives, including Peck's letter. He opens it. It's a simple drawing of a white tulip.