Each show gets a poster -- and the cast are posing in front of a new S.T.A.R. Labs facility (presumably the hangar-like building seen in teaser trailers for the event) that looks suspiciously like the Hall of Justice, home to the Justice League on Super Friends and, occasionally, in the comics.
There are also some interesting touches: alien invaders who appear to be Dominators appear next to Supergirl on the poster, presumably on her own Earth (or at least on her side of a breach which appears to connect her world to that of the other shows).
That's also yet another brief, teasing look at the Dominators, who have appeared in TV spots, but for only fleeting moments and with little or no real opportunity for fans to take in the effects.
Arrow, The Flash, and Supergirl will cross over with DC's Legends of Tomorrow next month in "Invasion!" a three-part crossover that starts in the final moments of an episode of Supergirl and then plays through the other series. Based on the 1988 comic book event miniseries Invasion! from Keith Giffen, Bill Mantlo, Todd McFarlane, Bart Sears, and the crossover -- with its marketing title "Heroes vs. Aliens" -- will closely track the plot of that storyline: aliens will be assembled by The Dominators and descend on Earth to bring an end to the "threat" of Earth's burgeoning metahuman community.
In the comics, the Dominion were secretly hoping to build their own super-race, and as a result created a handful of new superheroes in the course of the story.
In the TV version, rather than assembling a loose alliance of dangerous alien races, the Dominators will apparently be mind-controlling the aliens that join them. That's a revelation that suggests even the heroes might be forced to take one another (or at least the alien Supergirl) on at some point in the story.
"We utilize the crossover to ostensibly allow us to do whatever we want for the one hundredth," Amell told ComicBook.com. "So it's almost like the writers have a blank canvas."
Supergirl airs Mondays at 8 p.m. ET/PT; The Flash on Tuesdays at the same time; Arrow on Wednesdays and DC's Legends of Tomorrow on Thursdays. All four series air on The CW. The "Invasion!" crossover will air beginning November 28.