'''Season 2, Episode 19''':
!The Man from the Other Side
Walter has prepared himself to reveal to Peter the truth—that he is from the alternate universe—when they are called to a case. Two teenagers were found dead, evidently killed by shapeshifter agents from the alternate universe. Exploring the nearby area, they find a third unformed shapeshifter, which Walter takes to the lab to study. At the same time of the teenagers' death, they find a carrier signal in the local television. With Massive Dynamic's help, they determine that the signal emanated from the alternate universe during a brief moment that the two universes were in sync, and that the next point of synchronization would occur the next afternoon. Recalling Thomas Jerome Newton's previous attempt to bring a building from the alternate universe into the prime, the Fringe team believes they must stop this next attempt.

From the unformed shapeshifter, they learn of a name of its potential target, a doctor at a local hospital. They take the man into custody but find that he has yet to be harmed. Based on the need for three shapeshifters, Walter surmises that Newton is trying to use the same technology that he and William Bell had pioneered for crossing the universes, by placing three vibrational sources triangulated around the target. Walter begins to collect equipment to create interference with the vibrations to prevent the crossing. Another corpse is found, that of a bank manager that appears to have been killed by a shapeshifter. Realizing both men would have access to secured areas in their workplaces, they use these locations as two points on Netwon's triangle. From this, they are able to identify two locations where the transfer will occur. Olivia, using the information Walter has told her in private about his own crossing, identifies a collapsed derelict bridge over the Charles River, where the water would absorb the energy of crossing over.

The Fringe team converges on the bridge, where Newton has already started the process as the time of synchronization nears. As Olivia and the other FBI agent engage the two other shapeshifters in combat, Walter and Peter set up the interference device. Peter warns everyone, including Walter, back as he completes the interference device and the bridge, which still exists in the parallel universe, starts to appear with a man crossing it. The shockwave of its appearance disintegrates an FBI agent on the bridge with Peter but only sends Peter flying back and knocks him out.

Peter wakes in the hospital, learning from Olivia that they saw a man safely cross the bridge and taken by Newton. When Walter arrives to see Peter, Peter tells him he has realized the truth: because he, like the man on the bridge, was not affected by the same shockwave that killed the FBI agent, he must be from the parallel universe. Walter is unable to deny Peter's accusations. The next day, Walter is devastated to learn that Peter has discharged himself and has disappeared.


!Tropes
* InternalReveal: Peter finally learns the truth about where he came from.
* YoureNotMyFather: Peter to Walter, having worked out that he's from the alternate universe and as a result not this Walter's son.