In this episode, David Robert Jones (Jared Harris) employs former Cortexiphan subjects to create simultaneous earthquakes across the globe, leading Dr. Walter Bishop (John Noble) to hypothesize that the end of both the prime and parallel universes is imminent.

The combined Fringe teams agree that David Robert Jones is behind it, the quakes the result of stresses of bringing the two universes into synchronization so that he can collapse both of them.

When Lee reports this to Olivia Dunham (Anna Torv) in the prime universe, she suddenly recalls her Cortexiphan trials including fellow subject Nick Lane, and with the team's help, identifies that other Cortexiphan subjects are the epicenter of these quakes, linking to their parallel universe versions to achieve synchronization.

Believing that by taking in one of the Cortexiphan subjects they can stop the effects of another quake, a willing Lane from the parallel universe travels to the prime and is hooked to Walter's (John Noble) equipment.

With only hours left until the next set of quakes, as projected by a watch that Lane was wearing, plans are made to shut down the bridge before this time runs out.

Eventually, the Machine is overloaded and deactivated, and the parallel universe aspects of the room disappear; Walter is pleased to see Peter remains.

English director Charles Beeson, known for his work on the science fiction television series Supernatural and Terminator: The Sarah Connor Chronicles, directed "Worlds Apart",[1][2] his second episode of the season (the other being "Making Angels").

[3] In an on-set interview with The Huffington Post, actor John Noble explained that the episode's primary storyline concerned the bridge between the two worlds.

"[4] The reappearance of Nick Lane and his storyline helped resolve an earlier plot point from "Over There," in which he was recognized by Fringe agent Lincoln Lee before dying.

[5] Noble noted that "Worlds Apart' and the rest of the fourth season brought back a number of characters from the series' "rich history", including Sally, Nick, and David Robert Jones.

Entertainment Weekly writer Jeff Jensen noted that with the closing of the bridge, "Fringe effectively pulled the plug on the creative idea that defined the best years of the show.

"[8] Dave Bradley of SFX magazine highlighted the episode's acting as one particular positive element, praising Torv and Noble for "delivering perfect double performances.

"[2] Bradley also praised the scenes between the two Walters and two Astrids, writing that the "moments of connection make the decision to split the universes apart incredibly poignant.

"[2] Los Angeles Times writer Andrew Hanson also praised the Walter-Walternate scenes, explaining that after their seated conversation "you almost forget the two characters are played by the same actor.

"[11] His favorite part of the episode was when the two teams worked together; he gave particular attention to the Walter-Walternate talk, remarking, "The last scene between the two Walters is something that I’ve been waiting to see for years now, and John Noble nails both sides of the conversation.

[12][13][14] In a 2013 list, Den of Geek ranked "Worlds Apart" as the seventh best episode of the entire series, explaining that it "perhaps [is] a little rushed, but overall this is a satisfying conclusion to a number of major plot threads.