Crisis on Infinite Earths: Part One

"Everything we know... everything there is... and everything there ever was... is doomed."

- Pariah

The episode is the first part of the five-part crossover, ​.

Synopsis
The Monitor sends Harbinger to gather the worlds’ greatest heroes - Supergirl, the Flash, Green Arrow, Batwoman, White Canary, the Atom and Superman - in preparation for the impending Crisis. With their worlds in imminent danger, the superheroes suit up for battle while J'onn and Alex recruit Lena to help them find a way to save the people of Earth-38.

Trivia

 * Lex Luthor, Doomsday, General Zod, Eve Teschmacher, James Olsen, President Plastino, Alana, Beth Kane, Felicity Smoak and William Clayton were all mentioned, but do not appear.
 * The following episodes were referenced in this episode
 * The last time Supergirl saved Alana's pet extraterrestrial dragon, Spike from.
 * When Lex Luthor took control of the government in and.
 * Daxamites attacked Earth in and.
 * When James Olsen left the team in.
 * Lena Luthor betrayed Supergirl and Alex, and stole Myriad, only to blame it on Eve Teschmacher from the two episodes prior, and.
 * The Legion returned to the future without their ship in.
 * When Oliver and Sara Lance were presumed dead on the Queen's Gambit from the and Sara returned to Lian Yu in  and later to Star City in
 * When Sara and Ray changed the timeline.


 * This is the first episode of the series to not feature after being promoted to a series regular.
 * This is the first portion of a crossover to not feature.
 * This episode featured uncredited cameo appearances from Russell Tovey, Alan Ritchson & Curran Walters.
 * Ritchson and Walters reprised their respective roles as Hank Hall/Hawk and Jason Todd/Robin from Titans.
 * Earth-89, Earth-66, Earth-9, Earth-X, Argo City, and Earth-38 are wiped out by antimatter, leaving the status of many of their residents unknown.
 * As of this episode, Barry is the only living male character to star in an Arrowverse show after the deaths of Martin Stein and Rip Hunter in previous episodes & Oliver's  did.
 * Kara tells Clark, "Krypton isn't just a place, it's a spirit." This is a reference to the Marvel movie Thor: Ragnarok, in which Heimdall tells Thor, "Asgard isn't a place, it's a people."


 * Earth-16 Oliver Queen believed Sarah dead on the Gambit and they had never met prior. Whereas in Star City 2046, Oliver knew Sarah survived and referenced his conversation with Ray Palmer in Pilot, Part 1.
 * The Legion returned to the future with their ship. Mon-El and Winn Schott were shows inside the ship together, before returning to the future.

Production
Shooting for began on September 24, 2019.