When Walter discovers that Peter has traveled to the alternate universe, he and Olivia gather three cortexiphan children with the ability to transition safely between the two Earths.

Walternate asks Peter to help complete the device, while the fugitives Walter and Olivia are reunited with William Bell. Meanwhile, the alternate world's Fringe Division closes in on the invaders, and Peter must make a life-shattering decision.

The intensity of life “over there” accelerates as Olivia finds herself in mortal danger.

The beginning of the end is triggered when Walternate finds a way to wreak havoc “over here."