2. One Night in October

Complete story — includes the ending

The parallel universe Fringe division requests the prime universe's counterparts to help capture a highly intelligent serial killer, John McClellan, who has killed his victims by freezing their brain while experiencing the happier moments of their life. Fauxlivia wants Olivia to escort the prime universe's version of John, a professor in forensic psychology, to track down his movements, but without revealing the existence of the parallel universe to him. John helps to identify the killer's movements, but soon comes to realize that he is in the parallel universe, and escapes from the Fringe team. He ultimately meets with his doppelganger, and together they discover that though sharing the same life as a boy, their lives diverged due to the presence of a woman named Marjorie in the prime John's life that helped to soothe his homicidal tendencies. The parallel universe's John uses his equipment to experience the joy of Marjorie from his counterpart, but becomes distraught over his actions and kills himself. The experience traumatizes the prime universe's John, removing the memories of his experience in the parallel universe and of Marjorie. At the end of the episode, Walter starts hearing Peter's voice, begging for his help.