16. Peter

Complete story — includes the ending

Walter visits and explains to Olivia how, in 1985, he invented a window-like device that allowed him to witness events in the parallel universe. A younger Walter observes that, in the alternate universe, the other Walter (whom he nicknames "Walternate") is working on a cure for the genetic condition that is killing young Peter in both universes. One evening, however, Peter dies in Walter's arms. After the funeral, Walter witnesses Walternate synthesizing the cure. However, Walternate is distracted by an Observer and misses seeing the experimental result that confirms his success. Walter then decides to perfect the cure and administer it to the alternate Peter. Walter finishes the medicine and travels to Reiden Lake to open a portal. Nina, alerted by Walter's colleague, tries to stop him, fearing that breaking the barrier between the two universes will destroy both. In their struggle, Walter falls, but reaches the portal; Nina grabs him as it closes, and her arm is damaged by the energy. Walter's fall breaks the vial of medicine. He picks up Peter and returns to the other side to make more of the cure, but the frozen lake cracks beneath them and they fall through the ice. The Observer September saves them, explaining to Walter that Peter is important and must live. After curing Peter, Walter recants his original idea to return him because both he and Elizabeth, Peter's mother, do not wish to lose him again.