A man warps onto a commuter train, and all the passengers on the car die.
Walter writes a letter explaining everything to Peter.
The team arrives to investigate the commuter deaths. The entire car's power was drained, including all cell phones and lights. A witness claims to have seen a man exit the train, without speaking or acknowledging him.
Back at the lab, Walter learns the victims did not struggle for their last breath and the mitochondria in their cells where depleted.
They follow the video trail of the murderer to a cafe. They learn his name is Alister Peck and then the police raid his home. Peck teaches at MIT, and he's taken Einstein’s Theory of Relativity and used it to do whatever he is doing. Peck then arrives and walks into his apartment.
Peck explains to the Division that the people are not dead anymore but others will be shortly. Then he simply disappears and the scene returns to the beginning, to the original train full of dead passengers. The entire beginning of the episode repeats itself.
During the second search of Peck's apartment, Olivia has a moment of Déjà vu. Peter then finds a photo album with a picture of a woman. This time around, Peck never shows up.
Walter tells Olivia that she has written a letter to Peter.
At the Massachusetts Institute of Technology, the team learns Peck’s area of expertise was time travel. He also left his employment a year ago. Olivia finds out the woman in the picture was Peck's fiancée. A coworker also gives the team some of Peck's work that he was trying to get published.
Walter theorizes that Peck must be time traveling and Peck’s movement through time is what killed all those people.
Peck’s fiancée, Arlette Turling, was killed in a car accident 10 months ago. Peck must be going back to save her. The train deaths were only for a 12 hour jump, a 10 month jump would be catastrophic.
The team prepares to raid a lab at MIT, which is linked to Peck's cell phone bill. Walter asks to be allowed to talk to Peck, to get him to stop.
Walter disconnects his wire tap, and tells Peck how to correctly achieve the time travel. He also tells him about Peter, and that he never believed before in God but he now feels God is punishing him for what he has done.
Walter has asked God for a sign of forgiveness, a white tulip. If God can forgive Walter, then maybe Peter will be able to also.
Peck travels back to his apartment to readjust his calculations, and six more victims die. Peck then travels back six years, to the time when his wife died, just to tell her he loves her. They both then are hit by a car and die.
He never intended to save her, but only tell her he loves her one last time. With his death, none of the deaths ever take place.
Back at the beginning of the episode, and time, Walter receives a letter from Peck. It is a picture of a white tulip.