|"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.|
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 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 dimensional breach device 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 greet the Dark Arrow as Führer and he asks if the Nazis can get the device working. A female Nazi 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 (but neither has anyone drank Sara). 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. 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. Embarrassed, she leaves.
The next morning, Barry leaves quietly, 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 quietly leaves Sara in bed. 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 both she and Alex feel somewhat uneasy, and Kara eventually figures out that they slept together. Alex is embarrassed, but Kara assures her she did nothing wrong.
Inside the church, Mick happens to sit right next to Captain Singh. Barry meets with a serving girl, who is exited about the wedding and also tells Barry to say "I do". Barry wonders has he met the girl before. She claims they haven't. 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. Nazis 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 and take down Nazi soldiers while Kara takes on the woman, who has the same abilities as her. Jax and Stein merge as Firestorm and engage the Nazis. Sara and Alex take on the Earth-X version of Prometheus, and Oliver unsheats his portable bow, and takes on Dark Arrow, but not before saving Wells. Cisco tells Caitlin to introduce Killer Frost on these Nazis, and she does (and Frost complains about Caitlin's dress). During the fight, Oliver tells Cisco to breach him to the Dark Arrow, but finds he is equally matched against him. Cisco is later hit on the head by debris, and he falls unconscious. Kara and the woman are also equally matched, but the woman manage to beat Kara to the ground. Kara then claps her hands, sending a massive shockwave, that disorients the woman, and everyone else. She then beats her. Dark Arrow jumps to help her and the Nazis retreat. Sara and Alex defeat 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 was 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 emrges, 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 talkative mood, but Oliver says they'll make him talk.
On a roof top somewhere in Central City, Dark Arrow and the woman meet with Eobard Thawne/Reverse-Flash. He scolds the two for attacking recklessly and now Prometheus is 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 the woman breaks the fight. The woman 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.
- One of the songs played right before the wedding ceremony is Joseph Haydn's "Gott erhalte Franz den Kaiser", also known as "Kaiserhymne", an anthem to Francis II, Emperor of the Holy Roman Empire, which would later become the anthem of Austrian Empire and is the tune of the "Deutschlandlied", the national anthem of Germany, used by the Weimar Republic, Nazi Germany and the current Federal Republic of Germany.
- This is the first episode of to centrally take place on Earth-1. It is also the first time Alex has appeared in a crossover as she was absent (off searching for her father) when Kara met Barry for the first time in "" and was not involved with the and "" crossovers.
- This episode marks the first time since "" that Eobard Thawne appears as Harrison Wells in present day.
- At the beginning of the episode, after Kara defeated a Dominator, she said that the 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. He questions whether he'll have Spidey-Sense, but Stein tells him he won't, mockingly calling 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, previously sung in "".
- 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, Barry and Iris' future daughter in the Arrowverse, though until "", she was known 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 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 would not have had the capacity to reverse-engineer the German MP-44.
- Dark Arrow fires green arrows at the scientist, but he only carries red arrows.
- Barry and Iris were supposed to use "Runnin' Home to You" as their first dance, but was used as the bridal chorus for the episode. However, Barry and Iris may have simply changed their minds between their initial decision and their actual wedding.
- ↑ ""
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