Crisis on Earth-X is the name of the fourth annual shared multiverse crossover event between , , and . The crossover aired over two nights from November 27 to November 28, 2017. It served as a set-up for the animated web series
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.
A Nazi regime ruled the parallel world of Earth-X, where an archer known as Dark Arrow was the Führer. He seized a temporal gateway from the Freedom Fighters, which enables travelling to other universes. On Earth-1, Barry Allen and Iris West's friends, including Kara Danvers and Alex Danvers from Earth-38, came to Central City for Barry and Iris's wedding. Harry Wells, Cisco Ramon, and Caitlin Snow developed a serum to separate the Firestorm Matrix from Martin Stein and Jefferson Jackson. However, Jefferson was reluctant to give up being Firestorm. Oliver Queen re-proposed to Felicity Smoak, but she was hesitant about marrying him. The wedding ceremony was interrupted by invaders from Earth-X led by Dark Arrow, Overgirl, and Prometheus. After Kara injured Overgirl, and Alex and Sara Lance captured Prometheus, the Nazis retreated. Dark Arrow and Overgirl, who were Oliver and Kara's Doppelgängers respectively, discussed their next steps with Eobard Thawne, Barry's speedster nemesis who was previously presumed dead.
In S.T.A.R. Labs, Prometheus revealed himself as Tommy Merlyn's Earth-X doppelgänger, then took a suicide pill out of loyalty to the New Reich after taunting Oliver. Harry revealed that through his exploration of the Multiverse, he discovered that Earth-X was a dystopian world where World War II was not won by the Allied Forces. Dark Arrow, Overgirl, and Thawne sole an experimental sub-light generator, the Prism, from a research company. Oliver's team, along with Harry, Caitlin, Cisco, and Mick Rory, were in captivity at S.T.A.R. Labs after the Nazi forces occupied it. Oliver, Barry, Sara, Martin, Jefferson, and Alex were abducted to a concentration camp on Earth-X, while Kara was moved to S.T.A.R. Labs. Overgirl is dying from disproportionate insolation in her heart and Dark Arrow planed to use the Prism, powered by the S.T.A.R. Labs particle accelerator, to create artificial red sunlight that can weaken both Karas' invulnerability, allowing Thawne to conduct a heart transplant from Kara to Overgirl.
In the concentration camp, the heroes were rescued from execution at the hands of SS-Sturmbannführer by Ray Terrill and Leo Snart. Thawne prepared to operate on both Overgirl and Kara, while Iris and Felicity worked to rescue their friends at S.T.A.R. Labs. General Winn Schott, the commander of the Freedom Fighters, was determined to strand Dark Arrow and Overgirl on Earth-1 by destroying their temporal gateway. While posing as Dark Arrow, Oliver discovered the Nazis' doomsday device, a Timeship called Wellenreiter, a militarized equivalent of the Legends' Waverider. Oliver allowed the timeship to enter Earth-1 to avoid jeopardizing his cover, but he is ultimately exposed when he refused to kill Felicity Smoak's Earth-X doppelgänger, a concentration camp prisoner. The heroes struggled against both the Freedom Fighters' Red Tornado, deployed by Schott as a failsafe, and the Nazi forces, and the gateway's portal is opened at the cost of Martin being mortally wounded.
The heroes returned to Earth-1, and Iris, Felicity, Kara, and the others were rescued by the returned heroes and the Waverider crew. Jefferson was also affected by Martin's injury, so Martin used the serum to separate the Firestorm matrix and died from his wounds. Jefferson tells Martin's family of his fate; they, along with the Legends and Barry's team were devastated by Martin's death. His death spurred the heroes to declare war on Earth-X's Nazi forces. When the Nazis attacked Central City, the heroes countered their assault. Harry, who piloted the Waverider, destroyed the Wellenreiter after the heroes disabled its shield. Barry spared Thawne, who vowed to return. During the fight with Kara, Overgirl's solar radiation went nuclear and Kara carried her into space, where Overgirl exploded. Oliver killed Dark Arrow soon after. After Martin's funeral, Kara and Alex returned to Earth-38, Ray returned to Earth-X, and Leo decides to remain with the Legends. John Diggle, an ordained minister, officiated Barry and Oliver's weddings with Iris and Felicity, respectively.
- Stephen Amell as Oliver Queen/Green Arrow and Oliver/Dark Arrow
- Tala Ashe as Zari Tomaz
- Melissa Benoist as Kara Danvers/Supergirl and Kara/Overgirl
- Paul Blackthorne as Sturmbannführer
- Mehcad Brooks as James Olsen's Earth-X doppelgänger
- Christina Brucato as Dr. Lily Stein
- Tom Cavanagh as Dr. Harrison "Harry" Wells and Professor Eobard Thawne/Reverse-Flash
- Colin Donnell as Tommy Merlyn/Prometheus
- Franz Drameh as Jefferson "Jax" Jackson/Firestorm
- Victor Garber as Professor Martin Stein/Firestorm
- Rick Gonzalez as Rene Ramirez/Wild Dog
- Grant Gustin as Barry Allen/The Flash
- David Harewood as Director J'onn J'onzz/Martian Manhunter
- Juliana Harkavy as Dinah Drake/Black Canary
- Isabella Hofmann as Clarissa Stein
- Jeremy Jordan as Winn Schott and General Winn Schott
- Echo Kellum as Dr. Curtis Holt/Mr. Terrific
- Jessica Parker Kennedy as Nora West-Allen
- Chyler Leigh as Dr. Alex Danvers
- Keiynan Lonsdale as Wally West
- Caity Lotz as Captain Sara Lance/White Canary
- Jesse L. Martin as Detective Joe West
- Wentworth Miller as Leo Snart
- Danielle Nicolet as Cecile Horton
- Danielle Panabaker as Dr. Caitlin Snow/Killer Frost
- Candice Patton as Iris West-Allen
- Amy Louise Pemberton as Gideon
- Dominic Purcell as Mick Rory/Heat Wave
- David Ramsey as John Diggle/Spartan
- Emily Bett Rickards as Felicity Smoak and Felicity Smoak's Earth-X doppelgänger
- Brandon Routh as Dr. Ray Palmer/The Atom
- Patrick Sabongui as David Singh
- Maisie Richardson-Sellers as Amaya Jiwe/Vixen
- Russell Tovey as Ray Terrill/The Ray
- Carlos Valdes as Cisco Ramon/Vibe
- Chris Wood as Mon-El
- Nick Zano as Dr. Nate Heywood/Steel
- 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 episode "" by Barry as a hypothetical example of how various Earths in the Multiverse may differ from one another while explaining the concept of the Multiverse to Kara, James, and Winn.
- Ray Terrill/The Ray made his live-action debut in this crossover. His own animated series: premiered on CW Seed on December 8, 2017.
- Unlike previous crossovers, this crossover received its own special opening title card and did not have "previously on" at the start of each part.
- The only series regulars who didn't appear in the crossover were:
- Supergirl stated that there are 52 Earths in the Multiverse. Harry revealed that there are actually 53 Earths, but Earth-X is so horrible that no one goes there and therefore it does not have a formal designation. However, it is not indicated how or when Kara and Harry came to possess this information.
- This is contradictory to Martin Stein's statement in "" and Harry's statement in "" that there are "infinite" Earths as well as Red Tornado's statement in "" of Freedom Fighters: The Ray that there are "thousands" of Earths. Although, it's possible that Kara and Harry simply meant that there are 53 known Earths in the Multiverse.
- John Diggle, Ralph Dibny, Dinah Drake, Nate Heywood, Curtis Holt, Amaya Jiwe, Ray Palmer, Rene Ramirez, and Zari Tomaz were the only heroes not in attendance at Barry and Iris's wedding. That fact was referenced by Ray in "".
- There are two similarities between this crossover and the previous crossover ():
- This was the first crossover:
- To not reference Oliver shooting Barry which happened in the first crossover, .
- To feature Iris West and Harry Wells in all episodes. Previously, they had only appeared in episode of the crossovers.
- Where David Ramsey (John Diggle) didn't appear in every episode (unless "" is counted as a part of Invasion).
- Where Carlos Valdes (Cisco Ramon/Vibe) didn't appear in every episode.
- To feature Alex Danvers.
- So far, this was the only crossover where didn't serve as the first part of the event (unless "Medusa" is counted as a part of Invasion).
- This crossover introduced Nora West-Allen, Barry and Iris's daughter. However, since she is from the future, Nora did not reveal her identity to her parents and their family and friends until the events of "" several months later.
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