Over Here Astrid gets an unexpected visit from her Alternate, while Peter and Olivia track a killer using a toxin that has yet to be invented. Both universes collide in a case that pushes the boundaries of what is possible.

The dying Observer comes to see Walter and Peter, who must take extreme measures to learn what secrets he hides and determine if he knows where Olivia is. Meanwhile, David Robert Jones tries to force Olivia to use her cortexiphan abilities by torturing Nina.