At the end of the previous episode, Olivia Dunham (Anna Torv) was able to cross over back to her universe long enough to warn Peter Bishop (Joshua Jackson) she is trapped in the parallel universe.
Meanwhile, in the parallel universe, "Walternate" (Noble) plans on using Olivia's body in order for her alternate self to return home, as it would require a person of Olivia's mass to complete the transfer.