|"Things were looking a little grim, but part of me was, admittedly, a little excited for a "reunion" of old friends."|
- "This is sick."
- "You mean looking at your reflection and seeing only weakness? I agree."
- —Green Arrow and Dark Arrow
|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.|
- Stephen Amell as Oliver Queen/Green Arrow and Oliver/Dark Arrow
- David Ramsey (credit only)
- Willa Holland (credit only)
- Emily Bett Rickards as Felicity Smoak
- Echo Kellum as Curtis Holt/Mr.Terrific
- Rick Gonzalez as Rene Ramirez/Wild Dog
- Juliana Harkavy as Dinah Drake/Black Canary
- Katie Cassidy (credit only)
- Paul Blackthorne (credit only)
Special Guest Star
- Grant Gustin as Barry Allen/The Flash
- Victor Garber as Martin Stein/Firestorm
- Caity Lotz as Sara Lance/White Canary
- Tom Cavanagh as Harrison "Harry" Wells and Eobard Thawne/Reverse-Flash
- Chyler Leigh as Alex Danvers
- Dominic Purcell as Mick Rory/Heat Wave
- Candice Patton as Iris West
- Franz Drameh as Jefferson Jackson/Firestorm
- Danielle Panabaker as Caitlin Snow/Killer Frost
- Colin Donnell as Tommy Merlyn/Prometheus
- Melissa Benoist as Kara Danvers/Supergirl and Kara/Overgirl
- Adolf Hitler as Hitler (archive footage)
- Matt Letscher as Eobard Thawne/Reverse-Flash (archive footage)
The Earth-X Prometheus is revealed to be Tommy Merlyn. Oliver has a private talk with him, while others try to determine, where does these Nazis come from. Sara, Jax and Stein come up with a theory of time traveling Nazis, which could mean they are anachronisms, but it doesn't make sense, since they crashed Barry and Iris' wedding. Barry then thinks they are from another Earth. Kara says that there are 52 Earths, but none of them has a Nazi regime. Wells then says that there is a 53rd Earth, which is called Earth-X. It doesn't have an official designation, because it is so horrible no sane person would ever travel there. He then shows that Earth-X history followed same events as Earth-1, but with a critical difference. Stein realizes that on Earth-X the Nazis developed the atomic bombs before Americans, and used them to annihilate their enemies, thus winning World War II. Hitler continued his brutal reign until he died in 1994. Oliver tries to get through Tommy, but when Tommy manages to fool Oliver into thinking he was forced into this by the Führer, he kills himself with cyanide capsule.
Felicity calls Curtis and has him run a search for dimensional vortices supported with S.T.A.R. Labs systems. Oliver comes, and he talks with Felicity, why she refused his proposal. Felicity explains that while she loves Oliver, she is afraid that history will repeat itself, as she got shot after their last engagement.
Earth-X Oliver receives a signal that his best friend, Tommy, is dead. Thawne is upset about how their plan is not going as it should. Oliver slams Thawne to a pillar, forcing Earth-X Kara to step in, when Thawne threatens to rip Oliver's heart. Eventually they calm down, and Oliver orders Thawne to find the Prism. He leaves, and Kara encourages Oliver to trust in their plan, as he made it. They kiss, indicating they are married on Earth-X.
Caitlin, Wells and Jax try to come up with a way to track the Nazis, but it's not going so well, since Cisco is out of commission. Stein comes and asks to speak with Jax alone. Jax finally reveals, why he is acting the way he does. Once they disconnect Firestorm and Martin leaves, Jax feels he no longer will have what he really wants: Stein, calling him the closest thing he has for a father. Alex comes back to the Labs, and meets Sara. She asks is Alex okay, and she claims she is. Sara goes on the lift and thinks there's not going to be a thing between the two. Alex believes so, too. Kara comes and wonders, why Alex is like this. Alex explains that she thinks she made a mistake by making a one night stand with a strange woman, thinking it goes against the awesome thing she had with Maggie. Kara reassures her, that what she had with Maggie was real and awesome, but through her, Alex learned she is a lesbian and to be true to herself.
Iris rolls around in a chair and asks Felicity, how long is it going to take for the brainiacs to track down Earth-Xers. She doesn't know and tells Iris she is dealing with some Oliver stuff. Iris knows it's about Felicity refusing Oliver's proposal, as everyone heard her back at the rehearsal dinner. Felicity then gets a ping from a company building, which is under attack by Nazis. Iris goes to get Barry.
Outside the attacked building, Kara flies to the spot and Barry speeds there. They have to wait a while, as Oliver comes with his motorcycle. He then remarks he doesn't have super speed like them. The three heroes then come face to face with their enemies, who have stolen the Prism. Dark Arrow, Overgirl and Reverse-Flash unmask themselves, and all three heroes are shocked, as Oliver and Kara are going up against their doppelgängers, and Barry once again comes face to face with his arch-enemy. Thawne explains he used time-travel to escape his fate after Eddie Thawne killed himself, but nothing beyond that. Oliver-X remarks, that for both him and Oliver this is like looking in a mirror and seeing only weakness. Kara-X says the same to Kara, and gloats, how on Earth-X they formed a meritocracy after Hitler's reign, and Kara declares her a pervert. Thawne speeds up and Barry takes on him. Oliver takes an arrow and aims at Kara-X. She remarks it won't hurt her, but Oliver says this arrow will and shoots her. And the arrow does hurt her, as Oliver shot her with a Kryptonite arrow, shocking Kara. Oliver says he made them in case an evil version of Kara shows up. Kara-X attacks Oliver, but Kara jumps in between and blows her away with her freezing breath. Oliver-X runs after Kara-X, and she is pissed. He suggest she take her anger at a nearby building under construction. She uses her heat vision to weaken the building structure. The villains escape as Oliver, Barry and Kara move to rescue the workers and stabilize the building. They succeed.
Oliver, Barry and Kara return to S.T.A.R. Labs and Barry gives Alex the arrow Oliver used on Overgirl, and reveals that Dark Arrow and Overgirl are Oliver and Kara's Earth-X doppelgängers. Alex is shocked that there is another Kara, but Wells says there are actually 53 Karas, just like there are 53 Earths and 53 Kryptons. Alex goes to analyze the blood sample on the arrow, and quickly learns the blood has a massive amount of solar radiation within it, which could be a weakness in Overgirl. With this the team thinks they might be able to track her. Elsewhere, Thawne tells Kara-X that he is worried, that if the time comes, Oliver-X will choose his wife over the Fatherland. Kara-X claims Oliver will stay on the right path, but also assures that she will make sure he chooses the Fatherland over her.
Caitlin, Felicity, Wells and Stein are trying to locate Overgirl. Mick approaches Caitlin and asks her how Killer Frost appears. She uneasily admits it happens, when she is scared or angry. Mick tries to scare her, but Caitlin just remarks "Nice try." Wells manages to locate Overgirl. In the Time Vault, Barry looks at Thawne's records and remarks how seeing his face always brings back the memory of his mother's death. Oliver comes and encourages Barry and Iris to maintain their love, and Barry says Felicity will come around with him, but stops, when she appears. Felicity tells they found them. Alex, White Canary, Firestorm, the Flash, Supergirl and Green Arrow arm and prepare themselves for battle.
At the site, the heroes enter the Nazis location. Barry knocks all Nazis out, but soon they see they are surrounded. A fight starts and while Barry takes on Reverse-Flash and Kara takes on Overgirl, Alex says she can see enemy Flash and enemy Supergirl, which leaves everyone wondering, where is Oliver-X. Unknown to everyone, he's entering the S.T.A.R. Labs. In the Labs, the women are wondering, why Mick didn't go with the others. Wells encounters Dark Arrow, and manages to trigger the alarm, but Dark Arrow knocks him out. Mick tells Felicity and Iris to hide, while he and Killer Frost prepare to take on Dark Arrow. At the battle site, the heroes are fighting with everything they have, but Overgirl knocks Oliver down. He recovers and attempts to shoot her with Kryptonite arrow just as she is about to finish Sara off, but Overgirl catches the arrow. In S.T.A.R. Labs, Dark Arrow enters the Speed Lab, and is attacked by Mick. Dark Arrow easily disables him. He then remarks "I bet you're scared now!", but Dark Arrow claims he doesn't fear anything. Mick remarks he wasn't talking to him, and then Killer Frost grabs his bow and starts freezing it. She claims he didn't think this through, but he uses an electrical device to knock her out. Then, members of Team Arrow appear and take him on. However, he manages to beat them, and the Nazis also defeat the heroes at the battle site by unleashing a fully automated Metallo.
As the heroes recover, they find they have been shackled and they have collars on their necks. Thawne claims he could kill them, but Alex says they already would have, otherwise they wouldn't have the collars. Oliver-X appears, and Kara-X collapses, feeling intense pain. Through Oliver-X's encouragement that they found her, she manages to stand up. Thawne then reveals to heroes that Kara-X is dying. Alex realizes it's because of the massive solar radiation in her body. Kara-X confirms it by saying she flew too close to the sun. And in order to save her, she needs a new heart, which Supergirl can provide. That's why they stole the Prism, to use it in order to expose both Karas to red sunlight in order to perform the surgery for the heart transfer. They took others prisoner to use them as bargaining chips. Alex is terrified and Oliver claims that he will kill Oliver-X, but he claims he won't, since he is weak. Metallo unleashes a Kryptonite beam and knocks the heroes out.
In S.T.A.R. Labs, Felicity and Iris find the place is quiet, and wonder where everyone are. Felicity then wonders how can Iris maintain her life with aliens, Nazis and other villains constantly on them. She reveals that by waking next to Barry. Felicity then silences her, as they hear someone coming. It's a Nazi, and they back away. The women then witness the Nazis locking up Rene into the pipeline, and are worried, what has happened to everyone. Elsewhere, Oliver, Barry, Sara, Alex, Stein and Jax wake up somewhere, and Alex is terrified that Kara is gone. Sara assures her they'll get her back. People are being escorted somewhere, their hands bound. Jax claims this is not their Earth, and Sara claims they are on Earth-X, in a concentration camp.
- This is the first episode of the series to not feature David Ramsey (John Diggle).
- Felicity referred to Oliver of Earth-X as "Oliver in the High Castle", a reference to the television series and novel, Man in the High Castle, which also depicts a world where the Nazis won World War II. Similarly, the television adaptation explored alternate dimensions in its second season.
- This is the first episode to air on Monday.
- For Colin Donnell to do his guest spot as the Earth-X version of Prometheus, he wrapped up a full day of filming in Chicago Med, flew to Vancouver to shoot his scene, and then flew back to Chicago to resume filming Chicago Med the next day.
- When Oliver says, "You couldn't leave one for me?" to Barry Allen, it refers to when Nate Heywood said the same thing to Wally West in "".
- The Green Kryptonite arrow used by Oliver against the Kara of Earth-X is a reference to the arrow used by an older Oliver Queen to stop Superman in the storyline, Batman: The Dark Knight Returns.
- Harry says that there are "53 Karas" in the multiverse, assuming that every person has a doppelgänger in every Earth. This has previously been shown to not be true as Harry's daughter, Jesse, does not have a doppelgänger on Earth-1 and when Barry first arrived on Earth-38, he found out that himself and several other characters from and Arrow did not exist on Earth-38.
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