This week, millions of viewers will watch The CW's crossover between its four DC Comics-based superhero shows -- "Supergirl," "The Flash," "Arrow" and "Legends of Tomorrow" -- but if you're not a longtime DC Comics fan, you're likely not familiar with the story that inspired it.

This week's crossover, titled "Invasion!," is loosely adapted from a 1988-1989 DC Comics event series of the same name, from the creative team of Keith Giffen, Bill Mantlo, Todd McFarlane and Bart Sears. That story presented the alien Dominators, seen previously as "Legion of Super-Heroes" villains, as the leaders of an alliance of invaders, with Earth's superheroes stepping up to save their planet. According to "Arrow" and "Legends" executive producer Marc Guggenheim, the decision to adapt "Invasion!" came directly from Greg Berlanti, the executive producer of all four shows participating in the crossover.

"It was Greg's idea to do the Dominators," Guggenheim told press including CBR during a Q&A Monday at The CW's Burbank headquarters. "We all collectively wanted the superheroes to face an external threat. By external, I don't mean extraterrestrial, I just mean not a big bad from one of the shows, but rather a threat that came from outside of the shows. Greg just walked in one day -- 'Let's do Invasion!' I think [Andrew Kreisberg and I] both had the exact same reaction: 'That's totally awesome' and 'Oh my god, how the hell are we going to do that?'"

"Invasion!" provided a special challenge for Guggenheim and Kreisberg (also an executive producer on all four of CW's DC-based shows), who are both major comic book fans and comic book writers themselves -- Guggenheim was announced this week as the writer of Marvel's "X-Men Gold" series. Guggenheim was particularly excited to be adapting the work of Bill Mantlo, a veteran writer and the co-creator of Rocket Raccoon, who suffered permanent brain damage after being struck by a car in 1992.

"I'm a huge fan of ['Invasion!'], and I'm a huge Bill Mantlo fan," Guggenheim said. "That was Bill Mantlo's first work at DC Comics after decades at Marvel, so that particular series looms very large in my personal estimation."

According to both producers, the crossover was originally conceived as hewing closer to its source material, including in the visual depiction of the Dominators themselves.

"We originally had designed Dominators with the green robes that were very iconic of the comic," Guggenheim said.

"When we decided to do all the Dominators full CG, the robes had to go away, because the presence of the robes made animating them prohibitively impossible."

"There was discussions about the size of their circles, and how that was indicative of where they stood in the caste system," Kreisberg said. "I'm sorry some of that stuff went away, just because it was great flavor, and I think the people who are fans of the original comic book would have seen a lot more of our joy at adapting 'Invasion!' in there, but to get these down to the proper time, that's the kind of stuff that tends to fall by the wayside."

Following a short prelude in Monday's "Supergirl," "Invasion!" continues tonight at 8 on The CW with "The Flash," then tomorrow at 8 p.m. on "Arrow," before concluding Thursday at 8 p.m. with "DC's Legends of Tomorrow."