Fringe/Recap/S02 E10

Season 2, Episode 10:

Grey Matters
A group of Shapeshifters, led by Thomas Jerome Newton, break into a mental asylum and perform a brain operation on one of the patients that restores him to perfect sanity. The Fringe team investigates and discover two similar cases, and that each patient was cured by the removal of foreign brain tissue. Walter theorises that the tissue was added to the people's brains in order to preserve it (the tissue cannot survive more than a few hours outside a living organism), and that the pieces of tissue hold specific memories.

Olivia investigates the doctor who worked with each of the mental patients, the seemingly-fictional Dr. Paris, and discovers that he also visited one more mental patient: Walter, during his time in St. Claires. The team realises that the pieces of brain tissue came from Walter, and that if Newton is taking them, the pieces likely contain Walter's memories on how to create a door to the alternate universe. Just as Olivia and Peter are working this out, along with the fact that only Walter himself would be able to process the information stored in the tissue, Newton and the Shapeshifters kidnap Walter. They take him to the Bishops' old house in Cambridge and hook Walter up to a machine that temporarily reintegrates the removed brain tissue. A more aggressive and serious Walter refuses to answer Newton's questions on creating a doorway, but Newton forces him to talk.

When Fringe Division arrives at the Cambridge house, correctly guessing that Newton will try to use familiar surroundings to help Walter reaccept the memories, Newton injects Walter with a poison before fleeing. Olivia gives chase and captures Newton, but he forces her to let him go in exchange for information on how to save Walter's life. Newton escapes and Walter survives, but the removed pieces of brain tissue die and the memories they stored are lost forever. During a follow-up MRI, Walter has a flashback to being put under for the operation that will remove parts of his brain by Dr. Paris -- who is really William Bell.