Season one's Ability was considered for this article, which is also centred on Olivia's Cortexiphan abilities and is an early indication of the show's direction, but Subject 13 won out. It's a flashback episode in which Walter enlists young Cortexiphan test subject Olivia Dunham to help him return the kidnapped parallel universe version of Peter to his own world, while his doppelganger, Walternate, mourns the loss of his son. Your enjoyment of Subject 13 will likely depend on your thoughts on the implication that the lives of Olivia and the Bishops were always intertwined. Fringe liked to toy with that concept, most obviously when Peter is willed back into existence in the fourth season, but even the naysayers can appreciate the beauty of the opening scene of this episode, when a young Peter tries to smash through an icy Reiden Lake to return to his own universe. Subject 13 is filled with emotional moments, such as Elizabeth Bishop's attempts to bond with the resentful Peter, but it also contains one of the show's most memorable twists. Little Olivia tells Walter about her abusive stepdad and confesses that she did travel to the other universe, only for Walter to appear from behind her; she has been talking to Walternate, who now knows where his son was taken, and by whom.
