This season’s Arrowverse crossover, as announced at the end of last year’s “Elseworlds” crossover, is “Crisis on Infinite Earths”. Ordinarily, you could just watch the crossover without much regard for anything else. But given what’s already happened this season in The Flash and especially the events of the Arrow premiere I would say that the Crisis is already HERE. Seriously, go watch Arrow. The CW aren’t fooling around this time.
The DC comic version, published in 1985, was a twelve issue limited series along with tie-ins in some issues of their regular imprints. In the comics, a being known as the Anti-Monitor was destroying universes one by one. Another being called the Monitor assembled a group of superheroes from various universes to fight the Anti-Monitor and prevent the destruction of the multiverse.
What We Know About the Crossover
The crossover is going to involve all five (or is it six?) of the CW Arrowverse shows. As has become traditional, it will commence airing in early December. However, after the crossover episodes of Supergirl, Batwoman, and The Flash, everything goes on a five-week winter hiatus. The crossover then concludes with Arrow and Legends of Tomorrow.
Supergirl: Sunday, Dec. 8 at 8 p.m.
Batwoman: Monday, Dec. 9 at 8 p.m.
The Flash: Tuesday, Dec. 10 at 8 p.m.
[Winter hiatus]
Arrow: Tuesday, Jan 14 at 8 p.m.
DC’s Legends of Tomorrow: Tuesday, Jan 14. at 9 p.m.
Note that Legends isn’t even going to premiere until next year. They will have their crossover episode before their official season premiere.
There’s a separate thread that has casting news for the crossover. Since it lists who’s playing what, it may constitute spoilers for some. You can find it here: https://boards.straightdope.com/sdmb/showthread.php?t=879074
What’s Gone Before
The writers and producers have been setting up the Crisis almost since the Arrowverse began.
Arrow Season 1
Lyla Michaels, codename “Harbinger”, was introduced way back in the first season of Arrow. She, or possibly her doppelganger, will play an important role in the Crisis.
The Flash Season 1
The front page of Central City Citizen with the headline “FLASH MISSING - VANISHES IN CRISIS” has been a staple of The Flash since its first season. It was stored by Gideon in the Time Vault. Depending on the timeline, it is usually bylined by Iris West-Allen. Regardless of the timeline, until the current one, it’s always been dated to 2024 (see “Season Finales” below).
Elseworlds - Arrowverse crossover 2018
Last year’s crossover event only involved three shows - Supergirl, Arrow, and The Flash. It also introduced Gotham City and Batwoman to the Arrowverse. The Monitor appeared before John Deegan to give him the Book of Destiny and told him to rewrite as he saw fit. This was in fact a test by the Monitor, to find heroes for the coming Crisis. During the climactic battle, Oliver made a deal to serve the Monitor in return for him sparing lives of Kara and Barry. At the end of the show, Deegan is locked up in Arkham Asylum together with Roger Hayden, a.k.a. Psycho Pirate, who intoned, “Worlds will live. Worlds will die. And the universe will never be the same.” Finally, a title card announced “Crisis on Infinite Earths”.
Season Finales
Setup for the Crisis continued in each show’s season finale.
Arrow - Oliver said good-bye to Felicity leaves with the Monitor to fulfil his end of the deal. In a flash forward, future Felicity also goes away with the Monitor.
The Flash - The iconic newspaper changed its date from 2024 to 2019. The headline remained the same: “FLASH MISSING - VANISHES IN CRISIS”.
Legends of Tomorrow - Even though they weren’t part of Elseworlds, the Monitor appeared during their season finale. He simply munched popcorn while watching the Legends sing and dance their way to defeating the Big Bad.
Supergirl - The Monitor arrived on Earth-38, bringing with him another Green Martian who has some hatred for J’onn J’onzz. Then he apparently revived Lex Luthor.