Crisis on Earth-X, Part 1

"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 eighth episode of the sixth season of Arrow, eighth episode of the fourth season of The Flash and eighth episode of the third season of 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, the Nazi Germany and the current Federal Republic of Germany.
 * This is the first episode of Supergirl to centrally takes place on Earth One.
 * 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.
 * 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.