Fringe: Season 3, Episode 22

3 - 22: The Day We Died

Peter, with Olivia's help, enters the prime version of the machine, and experiences a vision of the future where the parallel universe has been destroyed and the same threatens the prime one. He opts instead to merge the two machine rooms, creating a bridge where inhabitants of both universes can solve their dilemma, before disappearing and being forgotten by both Walters and Olivias.