Two men dig up a box in someone's basemen. They open the box and go into a strange trance, begin bleeding from their noses and die.
Peter tells Broyles all about Walternate's plans to create a machine to destroy their world.
The Fringe team goes to investigate the deaths caused by the box. The victims were in some sort of trance before expiring.
Bolivia goes to see Newton, who wants to retrieve the box. Newton hires the two men to take the box and someone else then steals it.
Walter and Astrid begin dissecting the victims' brains. Peter then picks up Walter to take him to the reading of William Bell's will. He gives Walter a key to a safe deposit box with a note that reads: Don't be afraid to cross the line.
Bolivia begins searching a man named Blake's apartment, looking for the box. She then calls Newton and Blake writes down her name.
Walter confides in Astrid and she pleads with him to tell Peter his side of the story.
Walter then figures out how the victims must have died. Different sounds effect the brain in different ways. Harmonic music produces neuron activity. They begin looking for a silent and deadly ultrasonic music box.
Blake shows up at Bolivia's apartment with the box. He is deaf and that is why he didn't die. Bolivia calls Newton, secures the box and kills poor Blake.
Walter talks to Peter about how hard it was to lose Bell, which was nothing compared to losing his son. He explains that he only wanted to save him and it wasn't his intention to take him.
Newton next leaves the box with a homeless man. This man then opens the box and kills five more people. Peter decides to go looking for the box down the train station. Walter blows out Peter's hearing with gun shots.
Peter finds the box with the entranced man, just as his head blows up. Peter then recognizes the box as part of Walternate's dooms day machine.
The box was sound proof but now is broken so Peter disarms the box. Another train heads down the tunnel and Bolivia runs to save Peter just in time. Why was the box built on our side? Peter takes the box back to the lab to examine it.
Walter takes Bell's key and opens the safe deposit box. He then tells Astrid that William Bell has left him Massive Dynamic.
Bolivia types to Walternate letting him know Peter has the first piece and that he is actively engaged. Walternate then tells her to now begin working on Dr. Bishop.