|"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. They wonder of all the "undesirables" the Nazis have rounded up. Jax asks from one of the captives, what was his crime, and he says he loved the wrong person.
On Earth-1, the Nazis have rounded up nearly everyone in the S.T.A.R. Labs pipeline (including Cisco, Caitlin, Wells, Wild Dog, Mr. Terrific and Black Canary). At the same time, Kara has been strapped into a surgery table with artificial red sunlight from the Prism siphoning her powers. Thawne is preparing for the surgery, but it takes a while for Kara's powers to reach the point it can be performed. Overgirl then has a private chat with her doppelganger, and mocks Kara for protecting the weak, while she has taken on the belief that it is the purpose of the weak to fuel the strong and how she revels in being regarded as a god. Kara sees that Overgirl's ideology is not leading, but ruling. Overgirl agrees with that and tells Kara that her heart is worthless. But Kara counters it by saying that at least she has a heart. In the ventilation shafts, Felicity and Iris hear everything and they have to figure out, how to save Kara and others before the surgery is performed.
On Earth-X, the heroes are taken away by the leader of the concentration camp, who is revealed to be the Earth-X doppelgänger of Quentin Lance, known only as Sturmbannführer. He talks with Sara, telling that she once had a daughter like her, and killed her due to her sexuality. They are taken away to be executed, alongside the prisoner, Ray Terrill, whom Jax talked with. Oliver attempts to strike at the Nazis, but their collars act as shock collars, and they are easily put down. As the execution takes place, the heroes are suddenly rescued by Earth-X doppelganger of Leonard Snart, Leo Snart aka Citizen Cold. In the chaos, the heroes manage to break free and escape with Leo and Ray to the Freedom Fighters hideout, an Earth-X version of Arrowcave in Star City.
Back at Earth-1, Felicity and Iris attempt to free everyone from the pipeline, but before they succeed, they are just about to be spotted. Instead, Felicity sends a distress signal to the Legends.
The Freedom Fighters are led by General Schott, Earth-X counterpart of Winn Schott. He reveals the only way back to Earth-1 is through a breach facility, and he doesn't allow Earth-1 heroes to pass through it, as destroying it presents an opportunity to trap Nazi leaders, Dark Arrow and Overgirl to Earth-1, and give the Freedom Fighters a chance to liberate Earth-X. Alex tries to convince him to give them a little time for them to pass through, trying to convince him, that his counterpart on Earth-38 is one of Kara's best freinds, and he would do anything for her. The general refuses. Later, Alex tries to find a gun, intending to go it alone to save her sister. Sara finds her and swears Alex that they will not let Kara die, and reveals how she always wants to return to her former lover, Nyssa, but hasn't. But she gives Alex her support in saving Kara. Eventually, Ray and Leo manage to convince the General to give them the time they need to pass through the breach. The heroes put up a daring plan to infiltrate the facility, as Oliver can impersonate the Führer.
On Earth-1, the surgery is about to begin and Overgirl is put into a hibernation, while Kara is being kept awake. As Thawne begins, the power goes out. Dark Arrow orders a sweep. Felicity and Iris have managed to use the Time Vault computer to turn off the power. As the Nazis sweep the Labs, the women sneak to the medical room and free Kara. They try to take her outside to recharge, but all exists are blocked, forcing them to hide.
On Earth-X, Leo infiltrates the breach facility with Oliver and Oliver uses his appearance to trick the Nazis. He meets with the Sturmbannführer and Oliver witnesses as the Nazis send their secret weapon to Earth-1, the Wellenreiter, Earth-X version of the Waverider. Oliver had to let it go as to not blow his cover. Meanwhile, Sara, Alex, Stein and Jax sneak in to the facility, while Barry and Ray are ready to provide support. However, General Schott backs away on his promise and sends the Red Tornado to attack and destroy the facility, forcing Barry and Ray to take care of it before it reaches the facility. Sturmbannführer suspects Oliver is not the Führer, and puts him into a test: to kill a jew, who shared her rations with other prisoners in the camps. The jew is Felicity's Earth-X counterpart. Oliver shoots, but the gun is empty. With his cover blown, Oliver assaults the Nazis, and Alex and Sara engage the Nazis alongside Firestorm. Oliver jooins them, and the Nazis send everyone they have to fight the heroes.
On Earth-1, Thawne discovers, what Felicity and Iris did, and they and Kara are eventually found. As Felicity encoded the sequence to restore power,Thawne tries to make her tell. Felicity holds strong, but Thawne threatens to kill Iris, unless she gives the code. Kara, unwilling to let any of her friends get hurt, surrenders and tells Felicity to reveal the code, on the condition Thawne and Dark Arrow spare the others. In tears, Felicity reveals the code, and Kara is taken back for the surgery.
The fight on Earth-X keeps going. To activate the breach, two controllers must be used. Stein coerces Jax and him to separate, as he thinks they have a better chance to succeed like that. Jax manages to hotwire his end, but Stein gets pinned down by Nazi gunfire before he can reach the other device. Oliver, Sara, Alex and Leo take out many Nazis, but more come, while Barry and Ray attempt to deal with Red Tornado, eventually succeeding. Stein tries to run to the controller, but he is shot in the back by Sturmbannführer. The episode ends as Stein begins to bleed.
- 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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