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The CW's superheroes—The Flash, Green Arrow, the Legends of Tomorrow and the rest of the crime-fighting residents of Star and Central Cities—have fought off many a Big Bad from their Earth (and Earth-2). But this time, the threat is extraterrestrial—which is why they call in their friend Supergirl to help in 2016's big DC TV crossover event.

In the end of the Supergirl fall finale, we saw Barry (Grant Gustin) and Cisco (Carlos Valdes) visit Kara (Melissa Benoist) to request her assistance in battling their latest threat—which, as we'll learn in tomorrow's episode of The Flash, is alien in nature. What better way to fight off aliens than to enlist your own?

The Flash, Arrow, Legends of Tomorrow, Supergirl

The CW

The villains are an alien race called the Dominators, and they'll wind up on Earth (well, Earth-1) because of something to do with the metahumans. The storyline is based on the DC Comics crossover Invasion, and executive producer Marc Guggenheim says the idea for this year's event was based on an idea from DC TV super-producer Greg Berlanti.

"I think the way it came about was we all collectively wanted the superheroes to face an external threat. And by external, I don't mean extraterrestrial—I mean not a Big Bad from one of the shows, rather, a threat that came from outside of the shows," Guggenheim says.

The Flash, Arrow, Legends of Tomorrow, Supergirl

The CW

"Greg just walked in one day and was like, ‘Let's do Invasion.' I think [fellow EP Andrew Kreisberg and I] both had the exact same twin reactions: that's totally awesome and oh my God, how the hell are we going to do that? I'm a big fan of that particular series and I'm a big Bill Mantlo fan. That was Bill Mantlo's first work for DC Comics after decades at Marvel, so that particular series looms very large in my personal estimation."

You'll see how the Dominators are rendered on TV when the crossover continues on Tuesday's episode of The Flash, then on Arrow the next night before concluding with Thursday's episode of Legends of Tomorrow. The superhero shows all air at 8 p.m. on The CW.