Strange deaths by a mysterious shadow leads the Fringe Team to the scene where they learn that a Russian astronaut was possessed by an alien and his brother is convincing Dr. Bishop to solve the equation to free him.

After the extraordinary turn of events that shockingly left an imprisoned Olivia "over there," she fights to find her way home. Meanwhile, Peter and Walter try to move on with their lives unknowingly alongside alternate Olivia.

When the American Senator James Van Horn has a serious car accident, he immediately goes to surgery; however Newton invades the hospital and abducts him shooting the staff. There is a shootout with Broyles and Newton shoots Van Horn in the eye. The Fringe Division finds that the senator is a shapeshifter and they take his body to Walter's laboratory. He finds that it is possible to revive the shapeshifter and recover his data disc. But the alternate Olivia calls Newton and he sends the shapeshifter Ray to the hospital to retrieve the disc. When he returns home, Newton orders him to kill his family. However Ray is connected to his wife and son and refuses to follow the order. What will happen?

In the Season Four finale, the Fringe team is pushed to their breaking point as they desperately attempt to prevent a catastrophic event threatening the lives of everyone.