Fringe/Recap/S04 E03

Season 4, Episode 3:

Alone in the Dark
Fringe Division investigates a strange fungus after it starts killing people, and Walter comes to believe that it's symbiotically linked to a young boy who survived the first known encounter with it.

Meanwhile, after a psychological examination, Walter tries to demonstrate that he's functionally sane to avoid being sent back to St. Claires -- a task that's made all the more difficult by his hallucinations, which are getting worse. At the end of the episode, he attempts to give himself a lobotomy to try and stop the visions, but Olivia stops him at the last moment. He explains his hallucinations and Olivia reveals that she's been seeing the same man (Peter) in her dreams, and that she drew an accurate sketch and has been trying to identify him using the FBI databases, with no luck. After learning this, Walter becomes convinced that the man in real and they have to find him.