In Season 2 Episode 10 of "Fringe", what happens?

"Grey Matters" (aired December 10, 2009)

The team investigates a mental patient, who received brain surgery from a group of shapeshifters, curing his insanity. Viewing security footage, Olivia recognizes Thomas Jerome Newton (Sebastian Roché), the leader of the shapeshifters who was reanimated from the end of "Momentum Deferred" (episode 4). When similar cases occur, the team learns Walter had three pieces of brain tissue removed from his Hippocampus, which were stored in the brains of the cured patients. Knowing only Walter could comprehend the memories concerning how to open a portal to another reality, the shapeshifters kidnap him. By the time Walter is found, Newton has learned what he needs to know and escapes. As Walter undergoes an MRI, he imagines a past surgery, where he is being operated on by William Bell, who removed Walter's brain fragments and hid them in the patients' brains to prevent anyone else from gaining the information.

https://en.wikipedia.org/wiki/Grey_Matters_(Fringe)

I loved this show (for the most part) & was sad that it ended!