In the second part of the two-part season finale, Sacrifices will be made and both universes may never be the same again after Walter and Olivia visit the “other side”.

In the first part of the two-part season finale, Walter and Olivia travel to the parallel universe, and the anticipated face-off between Walter Bishop and William Bell occurs. Seth Gabel guest stars as the principal investigator in the Alternative Fringe division.

Peter teams up with a local law enforcement official, Sheriff Mathis, on a serial murder investigation with ties to Newton. Meanwhile, Walter copes with the possibility of being sent back to St. Claire’s, and someone from the “other side” pays a visit.

While Walter deals with some upsetting news, he tells Olivia’s niece, Ella, a fairy tale that includes musical performances by Olivia and Agent Broyles.

Two teenagers turn up dead, displaying wounds that are trademarks of the shape shifters. The Fringe division discover a shape shifting embryo and Walter returns to his lab to analyze the evidence, while Olivia and Peter go to Massive Dynamic in search for answers. Peter reveals a family secret and Walter tries to understand the motives of shape shifting mastermind Newton.

When passengers aboard a commuter train appear to have died a still death, it seems that a switch was flipped because all cell phones, mp3 players, laptops, batteries and bodies have been drained of power. As the Fringe team assembles at the bizarre crime scene, Peter remains suspicious that something is amiss with Walter, who is struggling to keep the unimaginable a secret. When the investigation leads them to Alistair Peck), a very powerful man who has tremendous energy with severe consequences, an ironic set of circumstances surface.

Olivia finds herself caught between keeping Walter’s secrets and her loyalties to Peter. Meanwhile, the Fringe division investigates the case of a woman who mysteriously drops dead from a disease she never even had and the team must discover its origins before it can claim more lives.

While explaining Peter’s other-worldly beginnings to Olivia, Walter flashes back to 1985. Peter’s mother is introduced and more details about “the other side” come to light.

A lone man survives a tremor that shakes a Manhattan office building to its core, leading the Fringe division to think that he is from “the other side”. Walter concludes that the earthquake was caused by something he and William Bell discovered years before and race to Jacksonville, the site of the pair’s previous experiments in order to avert another disaster, forcing Olivia to face her mysterious and traumatic past.

A wedding becomes a testing-field for a weaponized chemical agent that causes some wedding guests to suffocate from the inside out. As the Fringe division investigates, Walter uncovers a disturbing connection between the lethal experiment and the Bishop family. With the threat of more attacks mounting, Olivia, Peter and Walter race the clock in order to prevent more deaths.