"Peter" is the 15th episode of the second season of the American science fiction drama television series Fringe, and the 36th episode overall. Considered a keystone installment of the series, "Peter" is a flashback episode, told as Walter Bishop (John Noble) reveals to Olivia Dunham (Anna Torv) that his son Peter (Joshua Jackson) is really the Peter of the parallel universe. Walter explains the events that occurred in 1985 that led to this, and the impact it had on the parallel universe to be at war with the prime one.