Season 2, Episode 16:
- DECEMBER: (as he exits a late afternoon matinee of a 1985 sci-fi movie in the alternate universe) Their theories are fascinating.AUGUST: They are not theories. They are entertainment.
Walter explains to Olivia the circumstances surrounding Peter and his trip to the other side:
In 1985, Walter and William Bell had theorized the existence of an alternate universe, and created a window-like device to observe it. Though they use their observations for military benefits, Walter has a more personal interest in the alternate universe, to seek a cure for a genetic disease afflicting his son Peter. "Walternate", Walter's doppelganger in the other universe, is also seeking a similar cure, his Peter suffering from the same disease.
The prime universe's Peter succumbs to his illness and dies, and Walter and his wife Elizabeth mourn their loss, supported by Walter and William's friend, Nina Sharp and Walter's lab assistant, Carla Warren. Walter, through his window, shows Elizabeth the other Peter, and asserts they should feel comforted knowing another Peter exists. Later, while Walter watches Walternate explore other cures, Walternate is distracted by the arrival of September, and fails to see the telltale color change indicating a cure. Walter is able to recreate and stabilize Walternate's cure, and then decides to use untested equipment to cross over to give the cure to Peter. Carla tries to stop Walter, knowing the technology could damage the fabric of spacetime, and contacts Nina for help. Separately, September informs his fellow Observers that he may have made a mistake and assures them he will correct it.
Walter sets up his equipment on the frozen ice of Reiden Lake; Walter theorizes the frozen water will buffer the effects of the crossing. Carla arrives with Nina, and both try to talk him out of it. When Walter realizes William is not with them, he takes this as an implicit sign of William's consent to his plan, and activates the portal. Nina attempts to tackle Walter as he steps through, but instead part of her arm disappears in the portal as it closes; Carla rushes her to the hospital.
In the alternate universe, Walter finds the cure vial shattered from when Nina tackled him and he devises a new plan: to bring Peter back, administer a new batch of the cure, and return him. Encountering the alternate universe's Elizabeth, he pretends to be Walternate and claims to be taking Peter back to the lab for some tests. As Walter walks Peter back across the lake to the portal, Peter realizes that Walter is not his real father. They cross through the portal safely, but the ice has weakened and both fall through, losing consciousness. Walter wakes to find September driving him and Peter back to Walter's lab. September declares that "the boy must live", and leaves Walter to drive the rest of the way. At the lab, as Peter receives the cure, Carla informs him that William will see to replacing Nina's arm. Elizabeth arrives unannounced, and is overjoyed to see Peter, even though it is not her child. At that point, Walter realizes he will never be able to make himself return Peter to his proper universe—the pain of losing their child for the second time would be too great.
In the present day, Walter tells Olivia that his crossing is what caused the crack between the two universes, including the Pattern on their side, and leading to the oncoming "storm" that William warned Olivia about.
Tropes in this episode include:
- Acting for Two: John Noble, Orla Brady and Quinn Lord play the main universe versions of Walter, Elizabeth and Peter Bishop and also their Alternate Universe counterparts.
- Always Save His Son:Carla Warren: For the sake of one life, you will destroy the world.
- Apocalypse How:Carla Warren: We both know the amount of energy required to create a portal will forever ruin both universes.
- Character Title: The episode 'Peter' about the character Peter.
- Died in Your Arms Tonight: Peter's death
- Different World, Different Movies: Back to the Future starring Eric Stoltz.
- The '80s:
- Nina has a brick sized mobile phone.
- Asimov font for Title and Locations.
- E.T. Gave Us Wi-Fi: Well, the other universe, but aside from that, pretty much.
- For Want of a Nail: The Observers' concern with this is really highlighted.
- It's later revealed this is because of Butterfly of Doom. Any small change leads to a string of changes that completely alter the sequence of events. In their case, the Observers are concerned with their own existence, given this change caused a war that could destroy both universe, thus their existence.
- A God Am I:Walter: There's only room for one God in this lab, and it's not yours.
- #1 Dime: Peter's lucky silver dollar.
- Our Wormholes Are Different
- Portal Cut: Nina's arm.
- Curiously, however, her arm never quite goes through or even touches the portal. And it's not a clean cut, her arm slowly fades away.
- Portal Picture: The viewing device.
- Posthumous Character: Dr. Carla Warren and the Peter from this universe.
- Reality Is Out to Lunch:Carla Warren: Shattering the wall between universes would rupture the fundamental constants of nature.
- So, I was just measuring the elementary charge and it's now 1.802176565(35)×10−19. Who's been shattering walls between universes?
- Replacement Goldfish
- Ripple Effect-Proof Memory: The Observers.
- Soap Opera Disease: Peter's genetic condition
- Special Edition Title: See for yourself. Meant to be in a 1985 motif.
- Start of Darkness: We see how our Walter became what he is. Walternate's Start of Darkness comes later.
- Stock Quotes:"Now I am become Death, the destroyer of worlds."J. Robert Oppenheimer, quoting Hindu Scripture, after witnessing the first atomic explosion.
- Technicolor Science: Blue = Cure. Pink = waste of time. At least he didn't have to pee on a stick.
- Timeshifted Actor: Quinn Lord as Young Peter.
- Unwitting Instigator of Doom: The observer who distracts Walternate.
- Wham Episode
- Whole Episode Flashback
- Zeppelins from Another World: Showing the observation deck of the Empire State Building as a docking station for Zeppelins, as an Allohistorical Allusion.