It's back to the past for Walter on part one of the Fringe season four finale. He must head back there in order to, you know, save the world.

Fringe Season 4 Episode 21 Quotes

Jessica: What are you?
Walter: I'm human, what are you? Is this some sort of alien invasion? Are you part of a strike?

Peter: Three bedrooms and a dog run...
Olivia: Nursery?
Peter: Nursery?