SUPERGIRL [Episode Title: S03E08 "Crisis on Earth-X, Part 1"
THE EPIC FOUR-WAY CROSSOVER WITH "ARROW," "SUPERGIRL," "THE FLASH" AND "DC'S LEGENDS OF TOMORROW" BEGINS - Barry (guest star Grant Gustin) and Iris's (guest star Candice Patton) 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 (guest star Wentworth Miller), The Ray (guest star Russell Tovey), Felicity Smoak (guest star Emily Bett Rickards), Iris West and Alex Danvers (Chyler Leigh) to take on their most formidable villains yet. Earth's mightiest heroes - Green Arrow (guest star Stephen Amell), Supergirl (Melissa Benoist), The Flash (guest star Grant Gustin) and White Canary (guest star Caity Lotz) - lead their teams into battle to save the world.
ARROW Episode Title: S06E08 "Crisis on Earth-X, Part 2"
In S.T.A.R. Labs, a captured interdimensional villain from Earth-X, Prometheus-X, is revealed to be a parallel universe doppelgänger of Tommy Merlyn, and commits suicide out of loyalty to Earth-X's Nazi regime. Prometheus-X’s partners, Dark Arrow (Oliver’s doppelgänger), Overgirl (Kara Danvers’ doppelgänger), and Eobard Thawne steal a power source known as the Prism; Oliver believe that Dark Arrow seeks to build a neutron bomb with it. After a series of battles, most of the heroes including Team Arrow are in captivity at S.T.A.R. Labs after the Nazi forces infiltrate it, and Oliver, Barry and his allies are transported to Earth-X's concentration camp, but Kara is moved to another location. The villains reveal that Overgirl is dying and Dark Arrow plans to use the Prism to create artificial red sunlight that would weaken Kara's invulnerability, allowing a heart transplant for Overgirl but killing Kara in the process.
THE FLASH Episode Title: S04E08 "Crisis on Earth-X, Part 3"
Barry, Oliver, Sara, Alex, Martin and Jax wake up in a Nazi concentration camp on Earth-X, with power dampening handcuffs.
The leader of the concentration camp is revealed to be the Earth-X doppelgänger of Quentin Lance, who takes them away for execution, before they are saved by Citizen Cold (the Earth-X doppelgänger of Leonard Snart). Snart takes them to a resistance bunker, where the team meets fellow resistance fighters Ray Terrill and General Schott (Winn Schott's doppelgänger).
It is revealed the only way back to Earth-1 is through a temporal gateway in a Nazi facility, which Winn plans to blow up to destroy the enemy's doomsday device.
Oliver disguises himself as his Earth-X doppelgänger to infiltrate the facility, discovers the Nazi's doomsday device against the multiverse is a militarized timeship called Wellenreiter, and saves a prisoner (who is Felicity's Earth-X doppelgänger) from Quentin.
Barry and Ray battle the Earth-X Red Tornado, while the rest of the team enters the facility. In attempting to reactivate the portal, Stein is shot and gravely wounded.
Back on Earth-1, Thawne prepares to perform surgery on Kara to save Overgirl. Felicity and Iris try to stop him, but are captured.
DC'S LEGENDS OF TOMORROW Episode Title: S0308 "Crisis on Earth-X, Part 4"
Kara is saved from Thawne by Ray Palmer, while Nate releases everyone from the pipeline. On Earth-X, Stein activates the portal before succumbing to his injuries. Jax bonds with Stein to get him through the portal and back home, along with Barry, Oliver, Sara, Alex, Ray Terrill and Leonard Snart. Members of all the teams face off against Earth-X Metallo, while Felicity and Kara are cornered by Dark Arrow. Oliver gets Dark Arrow into a stalemate by threatening to kill Overgirl. Stein is kept alive by his connection to Jax, but it is slowly killing them both. Stein severs their connection and dies from his injuries; Jax delivers the news to Clarissa and Lily. Dark Arrow contacts the team to offer a truce, saying they will leave in peace if the team surrenders Supergirl. Oliver refuses and the team battles Nazis and the Earth-X doppelgängers, with Cisco, Wells, Felicity and Iris piloting the Waverider. Caitlin, Amaya and Zari infiltrate the Nazi's timeship Wellenreiter to disable its shield before Harry destroys it. Defeating Thawne, Barry is unwilling to kill him, allowing him to escape. Supergirl battles Overgirl, with the latter eventually going nuclear. Kara flies her into space, where she detonates. Oliver subsequently kills his doppelgänger. Cisco opens a breach, allowing Ray to return to Earth-X, with Snart choosing to stay. The team holds a funeral for Martin. The Legends, Kara, and Alex depart back to their own lives. Barry and Iris get Diggle to ordain their vows and officially marry, with Oliver and Felicity doing the same.