Walter finds out he may have brain damage on this episode of Fringe. "Letters In Transit" is the 19th episode of the 4th season.
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Fringe Season 4 Episode 19 Quotes

Broyles: Widmark. What did you do up there in the future to get yourself such a crap detail?
Widmark: I like animals.

I'll be a toe on a foot in a grave.

Simon