Crisis on Earth-X, Part 1

"Nazis? I hate Nazis."

- Kara, Oliver and Barry

"Crisis on Earth-X, Part 1" is the eighth episode of the third season of Supergirl, and the fiftieth episode overall. It aired on November 27, 2017. The episode is the first of a four-part crossover with the Arrow, The Flash and DC's Legends of Tomorrow.

Synopsis
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.

Trivia

 * 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 Supergirl to centrally take place on Earth One. It is also the first time Alex has been engaged in a crossover as she was absent (off searching for her father) when Kara met Barry for the first time and was not involved with the main Invasion! crossover.
 * This episode marks the first time since The Flash's 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, reference to the events of "Revenge of the Rogues"
 * 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 he's turning 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".
 * This episode marks the first appearance of Iris West and Joe West outside of The Flash.
 * 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 "Duet".
 * 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-One and the Arrowverse, as he was mentioned by Vandal Savage, who claimed he taught him archery.
 * This episode marks the first appearance of Nora, Barry's future daughter in the Arrowverse, though until a later time, she was known as Mystery Girl.