In the midst of writing a confession to Peter about his past, Walter is called into action on a case. A man has inexplicably materialized in the middle of a train car and killed everyone inside.
At the scene of the crime, the Fringe team notices that all the electronic devices on board the train have been drained of power. Broyles and Peter pull up surveillance footage from the train station, and get a clear look at their mysterious suspect.
Back at the lab, Walter examines the victims' bodies and discovers that they too have been drained of all their energy. Via surveillance footage, Olivia traces the suspect to a coffee shop and finds out that his name is Alistair Peck.
The team storms Peck's apartment and learn that the man is a brilliant professor of Astrophysics at MIT. Peck shows up at the apartment, and he asks them not to confiscate his research. Walter notices that Peck's body is entwined with copper wiring and he has thus created a temporal shield around himself. Without warning, Peck vanishes from the apartment and jumps back in time to the train sequence from earlier. This time around, Peck leaves a fingerprint while exiting the train car and Broyles is able to make a successful ID.
The team once again raids Peck's apartment, but all of his research has been preemptively removed from the premises. Peter, however, finds a photo album of Peck with his fiance as well as some homemade machine prototypes.
Olivia and Peter meet with Peck's former MIT colleague who gives them a copy of Peck's most current research -- a study of time travel. Walter processes Peck's work, and hypothesizes that the man's time jumps are sucking enormous amounts of energy from the areas in which he lands. The longer the jump, the more energy it requires. Olivia then discovers that Peck's fiance recently died in a car crash, and she suggests that Peck may be traveling back through time in order to save her. Walter explains that if their theory is correct, a jump back to the time of the car crash could create a devastating number of casualties.
The team locates Peck at a lab near MIT, and Walter goes in alone to negotiate. Walter tells Peck that he cannot jump back to the day his fiance died. Yet Peck says that he can do it in a way in which no one will die. Walter argues that it's not merely the casualties he's worried about, and he proceeds to tell Peck about how he stole Peter. Walter then confides that, ever since the day he crossed to the other side, he has been looking for a sign of forgiveness from God... in the form of a white tulip.
The FBI bursts in, and Peck jumps away. He lands in a large, open field, just moments before his fiance is about to be killed. Peck races down the street, and makes it just in the nick of time. Yet instead of pulling her out of the car, he gets in with her. Another car appears from out of nowhere and kills them both.
A year later, Peck's MIT colleague uncovers an unopened letter from Peck, addressed to Walter. She sends it to Walter, who has just thrown away the letter he wrote to Peter. Walter opens the envelope from Peck. Inside is the sign he's been looking for... a drawing of a white tulip.