Who are you? After sacrificing himself to save both worlds, Peter mysteriously returns to ours. But he has been forgotten - by Olivia, by Walter, by everyone. It's as if he never existed. At the same time, shape shifters controlled by an unknown master begin an onslaught of destruction, and now the two former enemy universes must cooperate to defy a common foe. Fringe's thrilling 22-episode fourth season continues the inspired series' synthesis of astounding phenomena, baffling secrets and dramatic, character-driven stories. And in this season, events may prove that the most powerful force in our universe - or theirs - is not a doomsday device, not a paranormal force, but human love.