Fringe/Recap/S02 E04

Season 2, Episode 4:

Momentum Deferred
A team of thieves has been raiding cryonic storage facilities and taking severed heads. Fringe Division is called in after one of the thieves is shot and killed by security during the heist and is discovered to be bleeding mercury: he's a Shapeshifter, one of the supersoldiers from the alternate universe. This discovery triggers severe headaches and flashes of memory for Olivia, who believes the thefts are related to whatever she learned during her trip to the other side. Peter recovers an intact shape-changing device, which Massive Dynamic's head scientist Brandon Fayette can use to repair the one that belonged to the first Shapeshifter they discovered and subsequently learn whose form it changed into last. To try and find a way to identify Shapeshifters, Walter contacts a former test subject, a woman whose psychotropicaly-induced insights originally tipped off Walter and William Bell to the Shapeshifters' existance.

Walter attempts to recreate the experiment that allowed her to see a "glimmer" around objects from the other side, using LSD and a hypnotic trigger in the form of a bell. However, when Walter rings the bell, Olivia falls into a trance instead and is finally able to remember what happened on the other side: her meeting with William Bell in his office at the top of the south tower. Bell tells Olivia that she was prepared by him and Walter to be a "guardian", someone who could help protect both universes and prevent catastrophe. He tells her that the Shapeshifters are searching for the head of their leader, which is marked with a particular symbol and can be found at a certain cryonics company's warehouse: if the Shapeshifters find this man, they will be able to open a door between universes. Before sending her home, Bell tells Olivia to contact Nina Sharp for help.

As soon as she's awake, Olivia visits Nina at the Massive Dynamic headquarters and tells her what she's remembered. Nina clarifies a phrase that Bell used, the "last great storm", which refers to the inevitable collision of universes as a result of Walter opening his original doorway: if the prime and alternate universes collide, only one will survive. Meanwhile, the Shapeshifter posing as Charlie learns that Massive Dynamic is in the process of reconstructing its shape changing device. Knowing this will mean its discovery, the Shapeshifter follows Olivia to Massive Dynamic and sends her a text claiming that Nina is the Shapeshifter and urging her to leave the building. As they leave, Olivia tells "Charlie" the location of the head that the Shapeshifters are searching for, only moments before being contacted by the Massive Dynamic labs and learning that Charlie is really the Shapeshifter. It knocks Olivia out long enough to contact the other Shapeshifters and tell them where to look, before Olivia fights back and eventually kills it. Fringe Division is too late to stop the Shapeshifters from raiding the cryonics lab and taking what they need. Broyles comforts Olivia, who is struggling with having to kill something that looked like Charlie.

At the end of the episode, the Shapeshifters attach the head they were looking for to a body, and it wakes up.