Fringe/Recap/S03 E08

Season 3, Episode 8:

Entrada
In the prime universe, Peter subtly quizzes Fauxlivia and confirms that she isn't the prime universe Olivia. She realises that her cover has been blown, gives Peter a paralytic and leaves his apartment with her laptop. When he recovers, Peter informs Fringe Division of Fauxlivia's identity and that she and him were in a relationship. They realise that Fauxlivia accidentally took Peter's laptop by mistake, and hand it over to forensics to crack her passwords. Fauxlivia visits the typewriter store and contacts the alternate universe, requesting an extraction. As the Fringe team try to figure out what Fauxlivia's next move is, Astrid spots a clue (the pastries that Fauxlivia kept bringing Walter) that indicates Fauxlivia had been visiting the Bronx, nowhere near her apartment, Walter's lab or the Federal Building.

Peter, Walter and Broyles go to the Bronx and look around. Peter finds the typewriter shop and recognises his laptop, left behind by Fauxlivia, and calls Fringe Division to investigate. They find the typewriter -- a quantum entanglement device -- in the back, and are able to read the last message sent from the other side, indicating that she'll be extracted from a train station at 4pm. As Fringe Division heads to the train station, Fauxlivia arrives and meets with a Shapeshifter, who injects tiny devices into both of her hands and the base of her spine. The Fringe team arrives and discover Fauxlivia holding a woman hostage, but when the woman doesn't respond to her daughter, Peter realises that she's a Shapeshifter and kills her. Fauxlivia is captured, but before being taken to a prison transport, she tells Peter that she's really grown to care about him during her time in the prime universe.

At the same time, Olivia is being held in solitary confinement in the alternate universe. Walternate plans on using her as part of a mass transfer to return Fauxlivia to the alternate universe, but not before cutting out her brain in order to further study her cortexiphan ability. When Broyles visit, Olivia tries to convince him that the two universes aren't at war (contrary to Walternate's arguments) and that she can help stop the conflict. He debates the issue and asks his wife for guidance, but eventually Broyles decides to help: he breaks Olivia out of her cell and brings her to the DOD lab. They discover that Walternate has been synthesising cortexiphan using what he's learned from her brain -- and what's more, Walternate emptied the tank, so Olivia can't use it to cross over. She has the idea to return to Walternate's old abandoned lab at Harvard University and use the old sensory depravation tank. As they arrive, Broyles reveals that the alternate universe Fringe Division is tracking them due to the tracker implanted in him, so as soldiers arrive to stop them, Broyles buys Olivia time to get in the tank. She successfully crosses over to the prime universe and emerges from the tank in Walter's lab, to Astrid's astonishment.

In the prison transport, the devices implanted in Fauxlivia's hands and spine activate. At the last moment, Walter realises that they're resonating rods, part of Walternate's mass transfer method for travelling between universes. Fauxlivia is transported back to the alternate universe, and the team discover -- to their horror -- that she's been swapped with the mutilated body of Broyles from the alternate universe.