Before the DC TV shows began airing, CW Chief Mark Pedowitz confirmed that there are plans of converging the storylines of not just Supergirl, Flash, and Arrow but to include Legends of Tomorrow as well. He said that he has a tremendous amount of faith in CW universe and it will be the biggest crossover event ever. In fact, recently Stephen Amell shared a picture on his Instagram page which shows the boots of Flash, Green Arrow, Supergirl, Atom and White Canary, and he wrote: “Coming Soon”. Marc Guggenheim (the executive producer) said: “It’s going to be an external threat that will involve Supergirl coming over to the Flash, then Arrow and Legends of Tomorrow.” Greg Berlanti said in an interview “You really re trying to run a single production across three different productions, but they run as three separate entities.”
He added “We have to figure out where we are borrowing one actor from where we are telling one cohesive story. Supergirl will participate but the story doesn’t actively begin there. The story will begin from Flash episode, goes to Arrow and Legends of Tomorrow.” He also indicated that it’s a 3-hour story and there are three different scripts of 180 pages long that will be sifted through for the ultimate crossover.
Earlier, everyone thought that the mega DC crossover will be based on a universe-altering, earth-shaking event with its origins in Supergirl season 2. But she is just there without any real motivation, and probably during the same time she is dealing with another villain with trans-dimensional powers. The mega-crossover will be based on Super Friends coming together to fight the Dominators, a technologically advanced alien race hell bent on destroying the potential threat of Earth’s meta-humans. The title of the episode is “Invasion!” across all shows except Supergirl where it’s titled “Medusa”.