|"Things were looking a little grim, but part of me was, admittedly, a little excited for a "reunion" of old friends."|
- "It's hard to believe a place like this actually exists, on any Earth."
- "I wish I shared your sentiment. But in all my travels to distant times, one thing that sadly remains constant is man's ability to feel hatred for other men."
- —Alex Danvers and Martin Stein on Earth-X
Barry and Iris's wedding brings the gang together, but things go awry when villains from Earth-X attack the ceremony. All of the superheroes band together with help from their super friends like Citizen Cold, The Ray, Felicity Smoak, Iris West and Alex Danvers to take on their most formidable villains yet. Earth's mightiest heroes - Green Arrow, Supergirl, The Flash and White Canary - lead their teams into battle to save the world.
Barry, Oliver, Sara, Alex, Martin, and Jax wake up in a Nazi concentration camp on Earth-X, with handcuffs and power dampening collars.
The leader of the concentration camp is revealed to be the Earth-X doppelgänger of Quentin Lance, who takes them away for execution, before they are saved by Citizen Cold (the Earth-X doppelgänger of Leonard Snart). Snart takes them to a resistance bunker, where the team meets fellow resistance fighters Ray Terrill and General Schott (Winn Schott's doppelgänger).
It is revealed the only way back to Earth-1 is through a temporal gateway in a Nazi facility, which Winn plans to blow up to destroy the enemy's doomsday device.
Oliver disguises himself as his Earth-X doppelgänger to infiltrate the facility, discovers the Nazi's doomsday device against the multiverse is a militarized timeship called Wellenreiter, and saves a prisoner (who is Felicity's Earth-X doppelgänger) from Quentin.
Barry and Ray battle the Earth-X Red Tornado, while the rest of the team enters the facility. In attempting to reactivate the portal, Stein is shot and gravely wounded.
Back on Earth-1, Thawne prepares to perform surgery on Kara to save Overgirl. Felicity and Iris try to stop him, but are captured.
- This is the first episode of the series to not feature Jesse L. Martin (Joe West).
- Barry quotes Leonard Snart's rules to making a plan: "Make the plan. Execute the plan. Expect the plan to go off the rails. Throw away the plan." Snart previously told Barry this in "".
- Harry exclaims "Rip van Ramon awakens" when Cisco wakes up, a reference to the short story "Rip Van Winkle" by Washington Irving.
- When traveling through the air conditioning vents, Felicity quips, "Lead the way, McClane", referencing an iconic scene in the 1988 film Die Hard in which protagonist John McClane does the same.
- Seeing the operation Kara was about to undergo, Felicity quips "And Kara has only a few hours before they go full Mengele on her". This is a reference to Josef Mengele, the Nazi doctor nicknamed Angel of Death due to his various inhuman experiments on the victims at the Auschwitz concentration camp.
- Iris tells Kara, "Come with me if you want to live", referencing the catchphrase from the Terminator franchise.
- When Harry is imprisoned in the pipeline, he begins throwing a ball against the wall of his cell. This could be a nod to The Great Escape, where Steve McQueen's character does the same in his cell.
- Wally West does not appear in this episode, nor is he credited.
- During the scene in which the Sturmbannführer is talking to Sara Lance, the song "Edelweiss" is heard in the background. This is a reference to The Sound of Music, which takes place in 1930s Austria. This may also be referring to The Man in the High Castle which primarily deals with a Nazi-controlled alternate earth and uses a cover of this song in its opening credits.
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