Obviously, Supergirl's episode -- "Medusa" -- was the show's midseason finale and thus only tangentially related to Invasion!, the kind of story that would be tagged "Invasion! - PRELUDE!" in the comics. The episode titles for The Flash, Arrow, and DC's Legends of Tomorrow are all "Invasion!"
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.
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; Arrowon Wednesdays and DC's Legends of Tomorrow on Thursdays. All four series air on The CW. The "Invasion!" crossover will air beginning January 9.