|"Things were looking a little grim, but part of me was, admittedly, a little excited for a "reunion" of old friends."|
|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.|
- Melissa Benoist as Kara Danvers/Supergirl and Kara/Overgirl
- Mehcad Brooks as James Olsen's Earth-X doppelgänger
- Chyler Leigh as Dr. Alex Danvers
- Jeremy Jordan as Winn Schott
- Katie McGrath (credit only)
- Odette Annable (credit only)
- Chris Wood as Mon-El
- David Harewood as Director J'onn J'onzz
Special guest stars
- Stephen Amell as Oliver/Dark Arrow and Oliver Queen/Green Arrow
- Victor Garber as Professor Martin Stein/Firestorm
- Jesse L. Martin as Detective Joe West
- Emily Bett Rickards as Felicity Smoak
- Caity Lotz as Sara Lance
- Tom Cavanagh as Dr. Harrison "Harry" Wells and Professor Eobard Thawne/Reverse-Flash
- Dominic Purcell as Mick Rory/Heat Wave
- Candice Patton as Iris West
- Franz Drameh as Jefferson Jackson/Firestorm
- Danielle Panabaker as Dr. Caitlin Snow/Killer Frost
- Carlos Valdes as Cisco Ramon
- Keiynan Lonsdale as Wally West
- Grant Gustin as Barry Allen/The Flash
Special appearance by
- Danielle Nicolet as District Attorney Cecile Horton
- Patrick Sabongui as Captain David Singh
- Christina Brucato as Dr. Lily Stein
- Isabella Hofmann as Clarissa Stein
- William Katt as a minister
- Anthony Ames as Angry Boss
- Robyn Bradley as Wedding Planner
- Mackenzie Gray as Greggor
- Paul Herbert as Scientist
- Jay MacDougald as Guard
- Andrea Pizarro as Manicurist
- Jeremy Schuetze as Rob
- Chris Walters as Norman Guard
In a Nazi-ruled universe, Earth-X, a resistance group comes under attack by Dark Arrow. He wipes out all resistance members, and also kills the Earth-X version of the Guardian, but not before he says someone will end the Nazi regime. Dark Arrow finds a device on the resistance hideout.
On Earth-1, heroes of Team Flash, Team Arrow and Legends contemplate about going to Barry and Iris' wedding, and they all agree to go. On Earth-38, Kara is fighting a Dominator and defeats it. Afterwards, she and Alex are brooding over their lives in Kara's apartment, until Alex sees the invitation to Barry's wedding. Initially both sisters aren't keen on going, but Kara decides they will, and takes her interdimensional extrapolator and opens a portal to Earth-1.
In a manicure shop at Central City, Iris, Caitlin, Felicity and Kara are having their nails done and discuss about Iris' wedding and others also suggest that Felicity would marry Oliver next (and the manicurist breaks her nail-maker on Kara's fingers). In a dress-shop, Barry and Oliver are proving their suits, and Oliver is happy Barry is taking the step, and he also wishes the same for him. In S.T.A.R. Labs, Wells and Cisco have managed to create a formula that can separate the Firestorm Matrix from Stein and Jax. While Stein is thrilled, Jax is not enthusiastic, as it would mean he won't have any powers.
On Earth-X, everyone greets the Dark Arrow as Führer and he asks if the Nazis can get the device working. A female Nazi, Overgirl, appears, flying like Supergirl, and tells that they have one day.
Just before Barry and Iris' rehearsal dinner, Jax leaves S.T.A.R. Labs and Caitlin walks in to talk with Stein, who is working with something. Caitlin takes also some credit for the cure, that was made to break up Firestorm. At the rehearsal dinner at CC Jitters, everyone greets each other, and Alex makes a closer acquaintance with Sara Lance, and they start drinking Scotch heavily. They are witnessed by Barry and Kara, with Kara claiming no one can drink Alex under the table (not knowing that no one has outdrank Sara either). Barry asks what's going on with Kara, and she reveals what happened with Mon-El, and Barry can't give her any advice on that. Barry then asks Kara to sing at the reception, and she agrees. Stein comes, and he privately reveals to Jax he was working on a compound, that will give him new superpowers after the Firestorm has been broken, but Jax is not thrilled about the possible abilities (it will allow him to stick to things). Joe holds a speech for the couple and points out that because of them, he openly reveals his relationship with Cecile. They toast, and outside, Sara and Alex are making out. Afterwards, Oliver proposes to Felicity, but she says no, claiming that while she loves him, she doesn't want to marry him, yelling it for everyone to hear when he continues to push for it. Embarrassed, she leaves.
The next morning, Barry leaves quietly, making Iris breakfast in bed for her to wake up to, and it is shown how everyone has slept (with Felicity wishing she could airsleep like Kara). Sara and Alex slept the night together, but when Alex realizes it, she is embarrassed and attempts to leave quietly. Everyone gather at the church, and Kara worryingly waits for Alex to show up, as she shouldn't be strolling on a different Earth. She eventually does, and claims she was okay, but can't bring herself to admit what happened last night. Sara arrives and, despite Alex trying to not bring it up, Kara eventually figures out that they slept together. Alex is embarrassed, describing what she did as "such a guy thing to do" but Kara assures her she did nothing wrong and describes Sara as awesome, though is less approving of her being an assassin, to which Alex adds she is a sassy assassin. Inside the church, Mick happens to sit right next to Captain Singh.
While waiting for Iris, Barry meets with a caterer, who offers him drinks. She is excited about the wedding, declaring it will be "one for the ages". Barry wonders if they've met before, to which the girl quickly claims they haven't. Before leaving, the girl urges Barry to remember to say, "I do".
As the ceremony begins, Kara sings Runnin' Home to You as Joe walks Iris down the aisle. The official ceremony begins, but when the minister asks if anyone is against this marriage, he is vaporized by a heat vision blast. Nazis from Earth-X, led by Dark Arrow, Overgirl and Earth-X version of Prometheus enter the church (with Oliver, Barry and Kara unanimously saying "I hate Nazis!"), and a fight breaks out. With no other choice, the heroes are forced to fight with their powers to save everyone. Barry and Wally use their speed to catch bullets, assist in evacuating the guests and take down Nazi soldiers while Kara takes on Overgirl, who has the same abilities as her, both landing solid blows on each other. Jax and Stein merge as Firestorm and engage the Nazis. Sara and Alex take down a group of soldiers together before taking on Prometheus; Oliver unsheathes his portable bow and saves Wells from Dark Oliver's arrows. Cisco tells Caitlin to introduce Killer Frost on these Nazis, and she does (and Frost complains about Caitlin's dress), launching icicles at them before engaging them with an ice sword while Mick covers her with his heat gun. During the fight, Oliver tells Cisco to breach him to the Dark Arrow, but finds he is equally matched against him. Kara and Overgirl continue to struggle to get one over each other until the latter throws the former throws the side of the church with enough force to launch debris everywhere, knocking Cisco unconscious, who is only saved at the last second from Nazis by Barry's intervention. Overgirl floats back into the church thinking she has won and tells Kara to stay down, however Kara rushes back up and claps her hands together, producing a powerful thunderclap, knocking Overgirl to the floor along with disorienting everyone else. Before Overgirl can recover, Kara grabs her and punches her hard in the stomach across the room, stunning her enough to prevent her getting back up. Dark Arrow jumps to help her and the Nazis retreat. Meanwhile, Sara and Alex have defeated Prometheus and take him captive.
Everyone gather at S.T.A.R. Labs and Prometheus is taken to the pipeline by Sara and Jax. Mick says everyone's memories about the attack are erased, so everyone's secret identities are safe. Stein and Caitlin examine Cisco. He has a concussion, but he'll be alright. Stein wonders why Caitlin is wearing her Killer Frost outfit, and she reveals that Killer Frost emerges, when she gets scared or angry, and she is certainly going to emerge before this is over. Barry asks Wally to protect Joe and Cecile. The heroes then wonder, who these people were. Stein claims it is unprecedented that they would really be from a Nazi regime, as it was destroyed over 70 years ago, and Oliver and Kara claim the enemies they took on were equal in power and skill to them. Prometheus is not in a talkative mood, but Oliver says they'll make him talk.
On a rooftop somewhere in Central City, Dark Arrow and Overgirl meet with Eobard Thawne/Reverse-Flash, wearing the face of Harrison Wells. He scolds the two for attacking recklessly and allowing Prometheus to be captured. Dark Arrow is revealed to be the Earth-X counterpart for Oliver, and he tells an opportunity came with the wedding. Eobard and Oliver nearly clash, but Overgirl breaks up the fight. Overgirl is revealed to be Kara's Earth-X counterpart, and she claims they will try again, and they will kill all the heroes of this Earth.
- This episode is tied with "" for being the highest rated episode of , with a rating of 8.8 on IMDb.
- This is the first episode of Supergirl to centrally take place on Earth-1. It is also the first time Alex has appeared in a crossover; she was absent (off searching for her father) when Kara met Barry for the first time in "" and was not involved in and "".
- This episode marks the first time since "" that Eobard Thawne appears as Harrison Wells in the present-day.
- Earth-X was briefly alluded to in the Season 1 episode "" where Barry was telling Winn, Kara, and James about the Multiverse. One scenario that Barry mentioned was an Earth where the Nazis won World War II.
- At the beginning of the episode after Kara defeated a Dominator, she mocked that Dominators are "so last year". This is a reference to the previous year's crossover, , in which the Dominators served as the main antagonists.
- Mick asks Caitlin if he kidnapped her once, referring to the events of "".
- Cisco refers to Harry as "Squidward", referencing the grumpy character Squidward Tentacles from the TV series SpongeBob SquarePants.
- Martin synthesizes a way for Jax to retain superpowers, albeit not his original ones. When he describes them, Jax suggests that Stein's trying to turn him into the Marvel superhero Spider-Man. Jax questions whether he'll have Spidey-Sense and when Stein tells him he won't, Jax mockingly calls himself "the Spectacular Sticky-Man".
- The music that plays during Barry and Iris's rehearsal dinner is the same music played in the episode "" when Oliver is about to propose to Felicity.
- When Barry asks Kara for a favor, he references the events of "".
- The song Kara sings is "Runnin' Home to You", the very same song Barry sang to Iris when he proposed to her in that episode.
- One of the songs played right before the wedding ceremony is Joseph Haydn's "Gott erhalte Franz den Kaiser", also known as "Kaiserhymne". This was an anthem to Francis II, Emperor of the Holy Roman Empire, and later became the anthem of the Austrian Empire. The song is now the tune of the "Deutschlandlied", the national anthem of Germany, which was used by the Weimar Republic, Nazi Germany, and the current Federal Republic of Germany.
- William Katt cameos as the ill-fated minister who oversees Barry and Iris' wedding. Katt is best remembered for starring in the comedic superhero series The Greatest American Hero in the early 1980s.
- When the Legends were in England, one of the knights says, "It's Robin Hood!" This confirms Robin Hood exists on Earth-1 and the Arrowverse, as he was mentioned to have been taught archery by Vandal Savage.
- This episode marks the first appearance of Nora West-Allen, Barry and Iris' future daughter in the Arrowverse, though until "", she was known out-of-universe as "Mystery Girl".
- When talking about the multiverse, Kara says that there are 52 universes. This is a reference to the New 52, the 2011 reboot of the DC Comics
- This is the first episode to feature Tom Cavanagh (Harrison Wells), Candice Patton (Iris West), and Jesse L. Martin (Joe West) outside of .
- The Yalta conference, as mentioned at the end of the episode, was a forum for the leaders of the United States, Great Britain, and the Former Soviet Union (Roosevelt, Churchill, and Stalin), to resolve issues of the post-World War II era. Yalta is a resort city in the contested region of the Crimean Peninsula, claimed by both Russia and Ukraine as of February 2014.
- Jax says, "Make America Aryan Again," in reference to the Nazis, a play on President Donald Trump's slogan "Make America Great Again."
- The symbol on Overgirl's chest and the emblem on Reverse-Flash's is the two stylized runes used by the Schutzstaffel, commonly known as the SS, the Nazis' security agency.
- Sara asking Alex, "How's your butt?" wasn't actually scripted; Caity Lotz improvised it and therefore, Chyler Leigh's reaction was completely genuine. However, the crew had to re-film her reaction because Chyler didn't know what to say and just laughed.
- Patton was given the chance to pick out Iris' wedding dress herself. She flew all the way out to Monique Lhullier in Los Angeles to find the perfect gown.
- Prometheus threatens Sara and Alex by saying, "One wedding...two funerals". At the conclusion of the crossover event, there are actually two weddings and one funeral.
- The resistance members at the start of the episode are seen carrying AK-47 assault rifles, which would not have been developed since the USSR had already been destroyed, and wouldn't have had the capacity to reverse-engineer the German MP-44.
- Dark Arrow fires green arrows at the beginning of the episode, but he only carries red arrows, and the later part of the scene showed him only shooting those.
- Barry and Iris were supposed to use "Runnin' Home to You" as their first dance, but the song was used as the bridal chorus in the episode.
- However, the couple may have simply changed their minds between their initial decision and their actual wedding.
- During the fight at the church, Oliver has a folding concealable bow, but it is never made clear as to where he would keep his arrows.
- When Kara performs a supersonic clap to subdue Overgirl, it shatters almost all the windows in the church. However, in a wide shot less than a minute later, all of the windows are intact.
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