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, shapeshifters 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 and baffling secrets.