Crisis on Earth-X

Crisis on Earth-X is the name of the 4 part crossover event between Supergirl, Arrow, The Flash and DC's Legends of Tomorrow. The crossover aired over two nights from November 27th to November 28th, 2017. The episodes that were involved in the crossover were:

"Crisis on Earth-X, Part 1" (Supergirl)
"Crisis on Earth-X, Part 2" (Arrow)
"Crisis on Earth-X, Part 3" (The Flash)
"Crisis on Earth-X, Part 4" (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.^[1]

Trivia

Earth-X is a world where Germany won World War II and Green Arrow, The Flash and Supergirl are evil. It was first referenced in the Supergirl episode "Worlds Finest" by Barry, who thought he made it up.
Ray Terrill/The Ray made his live-action debut in this crossover. His own animated CW Seed show, Freedom Fighters: The Ray, aired after the crossover event, on December 8, 2017.
Unlike previous crossovers, Crisis on Earth-X received its own special opening title card and did not have "previously on" at the start of each part.
The only series regulars who did not appear in the crossover were:
- Supergirl - Katie McGrath (Lena Luthor) and Odette Annable (Samantha Arias)
- The Flash - Neil Sandilands (Clifford DeVoe)
- Arrow - Willa Holland (Thea Queen) and Katie Cassidy (Earth Two Laurel Lance)
Supergirl reveals that there are 52 Earths in the Multiverse. Harry says that there are actually 53 Earths but Earth-X is so horrible that no one goes there and therefore it does not have a number. However, it is not indicated when and how Kara and Harry came to possess this information.
Ralph Dibny, John Diggle, Curtis Holt, Rene Ramirez, Dinah Drake, Ray Palmer, Nate Heywood, Amaya Jiwe and Zari Tomaz were the only heroes not in attendance at Barry and Iris's wedding. That fact was referenced by Ray Palmer in "Crisis on Earth-X, Part 4".