Season 2, Episode 22:
Over There
When a potential "breach" is detected in a Brooklyn theatre, the task of investigating falls to Fringe Division: the main team, consisting of Captain Lincoln Lee and agents Olivia Dunham and Charlie Frances, and the support team, which includes Colonel Phillip Broyles and agent Astrid Farnsworth. At the side of the breach, Lincoln sets up a quarantine canister but Astrid calculates that it won't be necessary since the breach isn't expanding into a vortex. As the team investigates, they discover a dead man covered in cancerous sores who has an identification card (instead of Show Me) and a twenty dollar note printed with Andrew Jackon's face instead of Martin Luther King Jr's.
A day earlier, in the prime universe, Walter is rewatching footage from a CCTV camera that shows Walternate and Peter travelling to the alternate universe. At the same time, Olivia is delivered something by the Observer: a drawing the shows Peter in the centre of a large machine -- a "doomsday device" -- with fire coming out of his eyes. When Walter sees the drawing, it jogs his memories of the Observer and being warned that Peter must not be allowed to return to the alternate universe, otherwise he may destroy one or both worlds. Olivia is determined to travel to the other side and rescue Peter, so she and Walter form a plan that involves reuniting several of the cortexiphan subjects and combining their power with Olivia's in order to safely cross over. Nina Sharp attempts to contact William Bell on the other side and arrange from him to meet the team, but can't confirm whether the message was received.
The journey is a success, but the other cortexiphan subjects' abilities start to go haywire as soon as they arrive. A man who can control illness breaks out in cancerous sores and crawls away to die, and a pyrokinetic has trouble keeping her power in check. The team makes their escape as the alternate Fringe Division arrives. While they walk to the park where William Bell is supposed to meet them, the alternate Fringe team is summoned to Liberty Island, the Department of Defence headquarters, to meet with Walternate, the Secretary of Defence. From the twenty dollar bill that the team scanned into evidence, Walternate has realised that a team has come from the prime universe. He informs Lincoln, "Fauxlivia" and "Scarlie" of the prime universe's existence: though they all believed the widely-held view (propagated by Walternate) that the breaches in the wall of reality, that lead to black hole-esque vortexes appearing randomly across the planet, are a natural phenomenon, Walternate informs them that the breaches are the result of the prime universe's incursion in 1985, which he considers an act of war. The Fringe team is ordered to find the team from the prime universe, and not to allow themselves to be manipulated by the other side's lies. Meanwhile, Peter is reunited with his real mother, and tells her a bit about the prime universe counterpart that raised him and ultimately committed suicide.
When the prime universe team reach the park, they are ambushed by Fringe Division. The rest of the cortexiphan subjects are killed, though not before the pyrokinetic blasts Lincoln and gives him serious burns. Olivia and Walter are separated, the latter being pursued by Fauxlivia: she manages to shoot him in the torso, but loses him in the trees. Bleeding, Walter manages to stagger to a hospital. Alone and with no other options, Olivia shadows Fauxlivia to her apartment and is met by William Bell, who wants her help in saving Walter. The episode ends with the reveal that Walternate has constructed the device depicted in the drawing of Peter: he removes one of the core components and leaves.