Olivia heads back to her alternate reality in the key episode "Momentum Deferred." She visit William Bell again during this hour.
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Walter helps Olivia revisit her alternate reality in the episode "Momentum Deferred." It's for her own good.
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Peter doesn't play a major role in the episode "Momentum Deferred." The hour focuses mostly on Olivia's return to her alternative reality.
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What is going on with Massive Dynamic. That's what Olivia Dunham wants to know.
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William Bell returns in the October 8, 2009 episode of Fringe. He's played, of course, by Leonard Nimoy.
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Fringe Season 2 Episode 4 Quotes

A storm is coming, perhaps the last and worst storm of all. And when it's over, I fear there will be little left of our world. The shapeshifters on your side are looking for someone to open the door between universes. and if they find him, there will be no stopping them

William Bell

Dear, I'm not certain that you're not simply a figment of my imagination.

Walter