Fringe Season 3, Episode 7: “The Abducted”. The real Olivia Dunham has been stranded in the alternate universe where she has been conditioned to believe she is actually her counterpart. Meanwhile, said counterpart is actually undercover in our universe.
The real Olivia Dunham is working as part of the alternate Universe Fringe division so solve cases on their side. A serial kidnapper known as “the candyman” has returned after a two year absence to abduct another child. The kidnapper extracts hormones from the abducted which he uses to make a sort of youth serum to regress his age. The unfortunate effect on the victim being massive internal degradation. Some of the kids even come back with cancer.
Christopher, the son of Agent Phillip Broyles was a victim of the kidnapper. Olivia overcomes Broyles objections and discovers some vital information hidden in Christopher’s memories.
Olivia makes a plan to break into the DOD compound on liberty island. There she plans to use the sensory deprivation tank to trigger her latent ability to travel between parallel universes. Olivia is able to temporarily travel back to the familiar universe but is pulled back when Walternate has her pulled from the tank. However, she is able to get a message to Peter Bishop who is having an affair with “Bolivia” who he believes to be the real Olivia Dunham.