|"Things were looking a little grim, but part of me was, admittedly, a little excited for a "reunion" of old friends."|
- "When you think about it... compared to the cosmos, one's life is but a blink of an eye."
- —Martin Stein to Jefferson Jackson
|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.|
Despite Stein getting shot, he gathers all his strength and manages to activate the device, opening the breach. Oliver, Barry, Sara, Alex, Leo and Ray defeat the remaining Nazis. In order for Stein to survive, he needs to get to the Waverider and have Gideon to treat him. Jax fuses with him to give him more time. The heroes return to Earth-1, and Ray and Leo join them.
In S.T.A.R. Labs, the surgery on Kara can finally begin. Due to pure sadism, Kara is still going to be kept awake during the procedure. As Thawne takes a scalpel and directs it towards Kara's heart, he suddenly stops. Dark Arrow wonders, why he isn't doing it. Thawne claims he is trying, but hasn't seen that Ray Palmer, shrunken with his Atom suit, is preventing him from cutting Kara. He enlarges, and knocks Thawne and Dark Arrow out. Kara is shockingly surprised, as she has never seen the full capabilities of Ray's suit. Ray releases Kara, while Nate frees the captives Cisco, Caitlin, Wild Dog, Mr. Terrific, Black Canary, Mick and Wells from the pipeline. Amaya and Zari take out the Nazis elsewhere. Oliver, Barry, Sara, Alex, Firestorm, Leo and the Ray also arrive and joins the fight. Ray, Felicity and Kara attempt to flee through the elevator, but it is guarded by Metallo. Felicity flees with Kara, and Metallo makes it's way into the Speed Lab, where it is destroyed with the combined might of Vibe, Killer Frost, Black Canary, Atom, Wild Dog, Mr. Terrific, Firestorm, Heat Wave, Citizen Cold, Steel, Vixen and Zari. Barry saves Iris. Dark Arrow goes after Kara and traps her and Felicity in the breach lab. He swears he will kill her unless she hands Kara. Felicity refuses and stands her ground, revealing how her grandparents didn't survive the Holocaust, but she will not back down, she'd rather die than let him hurt Kara. Oliver arrives at the scene, holding Overgirl with a knife at her throat. He swears he will kill her if Dark Arrow hurts Felicity or Kara. He then tries to convince his doppelganger, that S.T.A.R. Labs have some of the greatest minds on this Earth, and they can find a way to fix Overgirl's heart without the need to cut out Kara's heart. Overgirl urges her husband to kill Felicity, as she thinks Oliver is lying. As Dark Arrow is considering the possibility, Thawne speeds him and Overgirl away. Felicity and Oliver hug and kiss.
The heroes gather in the Waverider, and Stein is taken to Gideon to be treated with Caitlin helping the A.I. Many reunions and meetings take place in the ship: Kara and Alex are joyfully reunited, Mick meets his old partners doppelganger, Sara and Oliver discuss the situation.
Ray asks Caitlin, does Stein have any hope. Caitlin is not optimistic, as she and Gideon have tried everything. Jax goes to see Stein, and due their psionic link, Stein's injuries are also affecting Jax: if Stein dies so will Jax, as Jax is the only thing keeping Stein alive right now. Stein talks with Jax to let him go, as he has had a great life, but he is old now, and Jax still has a whole life ahead of him. Broken to tears, Jax tells Stein not to say things like that. Stein shows Jax the newly synthetic formula Gideon made, that can separate the Firestorm Matrix and tells Jax to give it to him. With an extremely heavy heart, after much convincing, Jax hands the formula to Stein and he drinks it. He thanks Jax for the adventure and dies immediately after. Jax cries openly over the death of his surrogate father. As Sara sees Jax crying, she goes to Stein to see him dead. Struck with the loss of her friend and comrade, Sara kisses his forehead. Stein's death affects everyone on the ship. Jax goes to Clarissa and Lily to tell them sad news.
The heroes then decide to gather for a one last battle against the Nazis, for Stein. Gideon informs Oliver that the Wellenreiter is sending a message. Dark Arrow gives the heroes of Earth-1 a chance to save this Earth: he and the Nazis will leave back to Earth-X, but Supergirl will come with them. Oliver declares the difference between them: he won't abandon his friends and cuts the transmission. As the captain of the Waverider, Sara commands the heroes to fight.
The Wellenreiter, commanded by Overgirl, starts bombarding innocents in Central City, while Dark Arrow leads the ground forces. As the Nazis cause mayhem, they are met by the heroes of Team Arrow, Team Flash, Legends, Supergirl, Alex, Citizen Cold and the Ray. An all out battle follows, in which Oliver takes on Dark Arrow, Barry takes on Thawne and the rest handle Nazis. Meanwhile, Cisco, Wells, Felicity and Iris fly the Waverider against the Wellenreiter. The only hope they have to destroy it, is to take out it's shields, but it's no use, as the ships are of equal power. Felicity says they need something more powerful to take it on. Kara is on the job, and flies after it. The Wellenreiter crew sees something aside Waverider following them. Overgirl commands to shoot it down, but they can't as it is too close. As Overgirl looks out from the window, she sees Kara, who openly challenges her. With great fury, Overgirl charges at Supergirl, starting the duel between the two Kryptonians. However, by charging at Supergirl, she unwittingly exposed the Wellenreiter. Felicity says someone has to get inside the ship to disable it's shields. Killer Frost, along with Vixen and Zari, uses her ice path to get inside the ship. The three women get inside, overpower the crew and manage to disable the shields. Cisco gives control of the Waverider to Wells while he breaches inside the Wellenreiter to rescue their friends. The Wellenreiter is destroyed, and the heroes gain an upper hand against the Nazis, with only Oliver and Dark Arrow still fighting an equal battle. Barry fights against Thawne across the city, and ultimately defeats him. Thawne goads Barry to kill him, but Barry refuses, and Thawne realizes yet again that Barry Allen is above killing. Barry tells Thawne to leave, and he does, abandoning his Earth-X allies, but also telling Barry they will meet again. Finally, Overgirl's excess solar radiation poisoning reaches its limit, and she starts to go critical. Wells informs Supergirl, and tells Kara she needs to fly her high above, outside Earth's atmosphere, otherwise she exploding will level Central City and more. Kara flies up, and Overgirl explodes as she reaches space. Dark Arrow witnesses the explosion, and is devastated over the death of his wife, falling to his knees. Oliver pulls an arrow, ready to shoot him. Dark Arrow rises with fury, swears to kill Oliver, but just as he assaults, Oliver shoots him in the heart, killing him. The Nazi invasion from Earth-X to Earth-1 is over. Kara drops down to Earth, and seemingly hits the ground hard, forming a small crater. But in reality, she was caught in arms by Nate, in his Steel form, and he quips that he figured it would take the Man of Steel to catch the Girl of Steel.
All members of Team Arrow, Team Flash, Legends, Kara, Alex, Leo and Ray gather at Stein's funeral, where Jax holds an eulogy. He breaks down and is devastated that he couldn't save him, as he only wanted to come home to Clarissa and Lily and little Ronnie. But Clarissa says Jax was his family too. As his grave is being filled, many people say their goodbyes: Clarissa says farewell to her loving husband, Lily says farewell to a great father, Sara thanks him for believing in her, Caitlin tells him to tell Ronnie she misses him. Mick also sheds tears for Stein, which Leo witnesses, but Mick claims it's allergies.
After the funeral, Cisco opens a breach for Ray and Leo to return to Earth-X. However, Leo decides to stick around on Earth-1 for awhile, and kisses Ray goodbye, promising to return. In the central park of Central City, Alex meets with Sara one last time and Sara encourages her to follow her instincts and believe there is someone else for her. The two women part as friends, with Sara boarding the Waverider for a next time traveling adventure. Kara arrives alongside Oliver, Barry, Felicity and Iris. Kara is sorry that Barry and Iris' wedding didn't go as they wanted, but they are going to do it lightly. Kara hugs Barry, Felicity and Iris, but with Oliver she does a fist bump, remembering Oliver is not one for hugs. However, Alex does hug him. Kara and Alex return through a breach to Earth-38. Barry and Iris then want to be married right now, and Oliver knows someone, who can help with that. Barry runs to Star City and grabs John Diggle from the Arrowcave and brings him to Central City (with Diggle vomiting yet again). As Diggle married his brother, Andy, and his wife, Carly, years ago, he has the authority to wed others. Barry and Iris go through their marriage vows with Oliver and Felicity as witnesses. As John is about pronounce the marriage, Felicity stops him, and asks that he marry her and Oliver right here and now as well. Oliver is surprised, but the war with Earth-X made Felicity realize she wants to be with Oliver and help him with William and everything else. They don't make vows, but they say "I do". John then pronounces Oliver Jonas Queen and Felicity Megan Smoak, Bartholomew Henry Allen and Iris Ann West as husband and wife.
- At the beginning of the episode, when Felicity tells Ray why Earth-X Nazis have taken over S.T.A.R. Labs, the last point Felicity says is "and they have a giant robot". Ray replies by saying "I've actually fought one of those." He then explains that in addition to shrinking, his suit enlarges. This most likely refers to the events of "", where Ray enlarges his suit, and himself, to enormous proportions to battle Vandal Savage's robot.
- While stopping the surgical blade, Ray quips "Mitts off Mengele". 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.
- Dinah likens Metallo to the Terminator from the Terminator franchise (much to Cisco's delight).
- Cisco compares flying the Waverider to playing Star Raiders on the Atari.
- When Supergirl says to Overgirl "General, do you care to step outside?", it is a reference to Superman II when Superman says the same thing to Zod.
- When asked by Cisco if he can handle piloting the Waverider, Harry quips that it's a "walk in the park, Kazansky", quoting the 1986 film Top Gun
- When Overgirl begins to go nuclear, Harry tells Supergirl that she has to fly her "Up. Up. And away", referencing the infamous Superman catchphrase "up, up, and away".
- When Kara asked Felicity and Ray how they know each other, they said that they used to date, referring to the events of the third season of .
- The picture that Ray was looking at after Martin's death is from the episode "".
- When Nate catches Supergirl, it is an homage to the cover of Crisis on Infinite Earths #7, in which Superman holds Supergirl (Nate even makes a reference to himself being a "man of steel").
- Former actress Susanna Thompson (Moira Queen) has an uncredited role as the voice of the A.I. of the Wellenreiter.
- Harry says to Cisco, "Cut the chatter, Red Two", a reference to Star Wars: Episode IV – A New Hope.
- Ray says "Mitts off Mengele", referencing Dr. Josef Mengele. He also references what Felicity said about Thawne in "".
- Earth-X's Snart makes an overture towards Sara, a callback to their implied relationship in Season 1 (although he later explicitly refers to himself as gay in "").
- This episode features the last appearance of present-day Martin Stein, following the events of this episode.
- Zari Tarazi debuts her alter ego's costume in this episode.
- There was a deleted scene in which Harry and Eobard met. Thawne intended to vibrate his hand through Harry's chest, but Harry filled his body with nanorobots that would reject contact with any speedster, since Harry did not want a speedster to harm him anymore. In this scene Harry calls Thawne an alternate version of him even if Harry disliked that his fellow Team Flash members referred to Thawne as Earth-1 Wells.
- When Diggle officiates Iris and Barry's wedding at the end of the episode, neither Barry nor Iris says "I do".
- This is ironic, considering that when Barry's future daughter approached him immediately before he and Iris tried having their wedding earlier in "" (prior to the Earth-X invasion), before she left, in her nervousness due to meeting her father for the first time, Nora told him "Good luck up there. Remember to say 'I do'."
- ↑ 'Arrow' Veteran Susanna Thompson Has a Surprise Role in "Crisis on Earth-X" - Comicbook.com
- ↑ Crisis on Earth-X preview: Zari finally has her costume and we’re loving it! - hiddenremote.com
- ↑ ""
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