The fate of humanity is on the line as Walter plans to make the ultimate sacrifice to stop the Observers before they were ever created.
To penetrate the Observers' headquarters and rescue Michael, their only hope for victory, Olivia must use her abilities and meet with some old friends.
While Walter tries to learn more about the mysterious Donald after Michael's revelation, Windmark undertakes his own quest for knowledge.
The team joins forces with Nina to recover a key piece of the plan that Walter created to defeat the Observers.
While Olivia and a recently-recovered Peter investigate a mysterious signal coming from a forest, Walter uses LSD to try and recall his memories of his plan to defeat the Observers.
Peter reaps the consequences of spying on an Observer leader. Meanwhile Olivia's search for another tape leads her to a woman who seems to be able to divine the future.
The fight against the Observers intensifies and a Fringe team member plans an attack on his own.
The Fringe team investigates a pocket reality occupying an apartment building, and Peter learns that he has the power to stop the Observers.
Peter takes extreme measures when the team learns of a way to stop the Observers from changing Earth's atmosphere and wiping out humanity.
After infiltrating a subway station to recover a vital piece of Walter's plan, the Fringe team meet up with an old friend... and must determine if he can be trusted.
The Fringe team meets with a group of humans who have taken refuge in the forest and seek to preserve humanity's history any way they can.
After Walter's mind is damaged by Windmark's telepathic probe, the team goes to Walter's old Harvard lab to find the information they need to defeat the Observers.
In the year 2036, Peter, Astrid, Walter, and Etta set out to find Olivia and free her from amber. Once they do, they discover she holds the key to finding the plans for a weapon to defeat the Observers... but Walter is captured and interrogated for the same information.