While The CW will attempt something no network has done before starting tonight with a four-night, four-series crossover between Supergirl, The Flash, Arrow, and DC's Legends of Tomorrow, the first part of that story -- tonight's Supergirl episode -- will only sort-of be part of the story.
The Flash, Arrow, and Legends of Tomorrow all have full episodes dedicated to the storyline and titled "Invasion!," the name of the 1988 comic book crossover storyline their title will loosely adapt. Tonight's Supergirl is called "Medusa," named for a Kryptonian project name-dropped at the end of last week's episode.
Supergirl, which aired on CBS last season and has not crossed over much with the characters of the "Arrowverse," has been a major part of marketing the "Heroes vs. Aliens" event, so some fans have (not entirely unreasonably) wondered why her series' involvement in the crossover itself feels like it was a bit downplayed.
During a screening of The Flash, Arrow, and Legends of Tomorrow that took place earlier today in Los Angeles, executive producer Andrew Kreisberg, who works on all four of the shows but seems to be most hands-on with Supergirl these days, explained the approach, saying that it was almost exclusively a practical decision.
"All of the other episodes — Flash, Arrow, and Legends — all have a subsequent episode to the crossovers, but with Supergirl it was the midseason finale, so we wanted to make sure that episode eight of Supergirl spoke to what had been happening in the first seven episodes of the season," Kreisberg said. "And then just, logistically speaking, last year we did two shows with a sprinkling of Legends. This year we went to three shows. The idea of going to four full shows, I'm not sure if we could have pulled off logistically speaking."
Similarly, Kreisberg pointed out, Kara was not in much of last week's episode (she was captured by Cadmus, and other characters took the stage) because they were freeing up five days on series star Benoist's schedule to appear in the other series.
Arrow, The Flash, and Supergirl will cross over with DC's Legends of Tomorrow this week 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.
NEXT: The Flash, Arrow, Supergirl, Legends of Tomorrow Crossover Trailer / First Look At The Dominators / Invasion! Crossover Promo Photos / Potential The Flash Spoiler Spotted On Set / Will Joe West Die During The Invasion! Crossover? / Enter To Win An Ultra-Rare Invasion! Crossover T-Shirt
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.