The episode is the third of a four-part crossover with the eighth episode of the third season of Supergirl, eighth episode of the sixth season of Arrow and eighth episode of the third season of DC's Legends of Tomorrow.
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 power dampening handcuffs.
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.
- 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." He previously told Barry these in the third season The Flash episode "". which Leo thinks is a bad idea, as he always has a plan referencing his Prison Break character Michael Scofield.
- This is the first episode of The Flash where Joe West does not appear.
- Wally does not feature in this episode at all.
- The Ray confirms that he is from Earth One, the reasons as to how and why he ended up on Earth X is unknown.
- The Ray and Citizen Cold are in a relationship together.
- This is the first episode of Season 4 in which Clifford DeVoe doesn't appear or even mentioned at all.
- This episode marks the first time since "Who is Harrison Wells?" that Paul Blackthorne appears on The Flash.
- This is the second episode of the Flash where Jeremy Jordan (as Winn Schott of Earth-X) appears in, the first being "Duet", where he appeared as Grady.
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