Fringe Season 1 Episode 20 “There’s More Than One of Everything”
Nina Sharp recovers from the attack and reveals that Jones stole an energy cell in her mechanical arm. He intends to use it to power a device that would open a doorway into a parallel universe and confront William Bell. Peter finds Walter in a beach house, retrieving a device that would close any doorway that Jones opens. Walter explains to Peter that he lost someone dear in the past and had to go to another reality to bring it back.
The trio intercepts Jones while he was opening a window, and Peter kills Jones by triggering Walter’s device, with the door closing on Jones while he was still halfway through it. At the end of the episode, it is revealed that Walter’s real son died back in 1985, and that the Peter they are with right now came from a parallel universe. Meanwhile, Olivia is taken to the parallel universe and meets William Bell.