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...