Elseworlds, Part 3

The episode is the third part of the three-part crossover with the ninth episode of the fifth season of, and the ninth episode of the seventh season of.

Synopsis
Supergirl, The Flash, Green Arrow and Superman engage in the battle of their lives.

Trivia

 * The episode was previously titled "Elseworlds: Hour Three".
 * Barry breaks the fourth wall in this episode; he says "this has gotta be the craziest thing you've heard in the past three hours"; a reference to the fact that the Elseworlds crossover is split into three hour-long installments.
 * The opening monologue in this episode was narrated by John Deegan. But for the last line, he says it after transforming into Superman.
 * Near the end, Deegan's face is shown disfigured in black and white almost like a skeleton, similar to his comic counterpart's appearance.
 * When John Deegan becomes Superman and rules the earth, this is a reference to the plot of the video game Injustice: Gods Among Us where Superman is a tyrant that rules the earth.
 * When Deegan fought Superman in a building, a civilian calls him Bizarro. This is a nod to Bizarro, who is female in the Arrowverse.
 * In the comics, Doctor Destiny is associated with Dreamstone, artifact created by Dream of the Endless, while his Arrowverse counterpart uses Book of Destiny, artifact created by Dream's older sibling Destiny.
 * When Barry and Oliver are looking for Cisco, Barry references the season 3 premiere Flashpoint when he rewrote reality and Cisco wasn't much help.
 * Kara correctly guess Lois and Clark are having a baby, a reference to Jonathan Samuel Kent, who first appeared in DC Comics' Convergence event in the miniseries Convergence: Superman #2 (July 2015). Jonathan is the biological firstborn son of Superman/Clark Kent and Lois Lane and is half kryptonian/human
 * Gary says Oliver and Barry are his heroes, "Legends", a reference to Legends of Tomorrow who are not part of the annual crossover this year, unlike the previous two years.
 * While in the Fortress of Solitude, Superman presses a piece of coal into a diamond to put it in a ring and propose to Lois Lane, resembling Smallville's 100th episode ("Reckoning"), when Clark Kent did the same to propose to Lana Lang. The aforementioned scene itself is a reference to Superman III, when Superman presses a piece of coal to give Lana Lang a ring after she tells him she has to sell hers.
 * Superman says to John Deegan as Superman, "doctor can I have a word", a similar quote to Superman II, when he says to Zod, "General, would you care to step outside?"
 * Furthermore, it resembles Kara's quote to Overgirl in"​"​ to lure the evil doppelganger into a fight.
 * One of the scenes of S.T.A.R. Labs shows it to be under Nazi control with a red sky, a possible nod to the previous crossover event, Crisis on Earth-X.
 * This episode introduced us to Earth One doppelgänger versions of James Olsen and Alex Danvers.
 * The crossover ends with the title of the next crossover, Crisis on Infinite Earths, which will premiere in the Fall of 2019.
 * At the end Roger Hayden says "Worlds will live, worlds will die and nothing will ever be the same." The exact lines Psycho Pirate said in the comic Crisis on Infinite Earth.
 * Flash and Supergirl dying in Superman's vision of the future is a reference to the original Crisis on Infinite Earths story-line, where both heroes sacrifice their lives to save the universe.