Any episode which features a reluctant collaboration between the two universes is guaranteed fun, but Worlds Apart is also our farewell to the parallel universe, and it was startling to find how much we cared. Yes, it's fun to see the return of Cortexiphan subject Nick Lane and the coordinated efforts of the two Fringe teams to stop David Robert Jones, but Worlds Apart is best remembered for its shattering final scene, in which the bridge between the universes is closed for good. Walternate admits that Peter belongs with Walter and that he is proud of the man his son has become. Olivia voices her grudging respect for Fauxlivia, who returns it. Peter chooses to return in the prime universe to Walter's obvious delight, while Lincoln Lee leaves his original home to stay by Fauxlivia's side. It's a hugely gratifying episode for fans who'd followed the parallel universe's development from unseen source of strange happenings to a fully realised alternate world, and it's a testament to the phenomenal cast that each departure feels real, although most of their faces returned next week.
