Try as you might, there are some marks that can never be scrubbed out entirely. There are some people who leave an indelible impression on our souls which remains long after they've gone, an afterimage burned onto our retinas, an echo of a cry for help, a sigh, a plaintive wail, or a whispered declaration of love.
Within the world of Fringe, Peter Bishop no longer exists. We saw him blink out of existence at the end of the third season finale, flickering before our eyes as two universes forgot all about him. Nature, of course, abhors a vacuum, so time and space rush to fill the void left behind when an item is plucked out of the timestream.
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The season opener ("Neither Here Nor There") contains a rather ordinary procedural plot, but it also reintroduces us to the two universes, and to changes that have occurred as a result of Peter's non-existence. Some of these changes are slight, and some are rather large. The dead walk again as the living, memories are altered, personalities shifted as a result of Peter not being in the mix since the series began.
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But it's the second episode of the season ("One Night in October") that brilliantly showcases what Fringe is capable of: emotionally resonant stories with sci-fi trappings that are intensely character-driven explorations of the human heart. This is very much the case with the largely Over There-set installment which finds the Fringe Division attempting to entrap a vicious serial killer whose methods for spreading death are rather unique, yet also connect to the wider philosophical issues at play here. Are we the sum of our experiences? Do our choices define us? Can we remember when those memories are cruelly ripped away from us?