The CW have already been advertising four-series of crossovers between DC Comics properties. DC dives into the Arrowverse's that adapts "Invasion" comics, but the "Supergirl's" finale "Medusa" will serve as a prequel for a full showdown opposite to the Dominators. CW's crossover will start the midseason finale of "Supergirl" and the continuation mostly of the week on "The Flash," "Arrow," and "Legends of Tomorrow."
According to IGN US, the storyline of "Medusa" is the end of the episode. The main reason why Supergirl is to focus on "Supergirl" instead of being crossover because the episode is double as the show falls finale. The three shows - Flash, Arrow, and Legends have this all subsequent episode for the crossover but on Supergirl they wanted that the episode 8 will be like in the first seven episodes not just the tie in.
Kreisberg said that that show was not able to pull off the expanding from the last year's two show crossover into a full four show crossover but it did a three-show crossover with a little teasing in fourth. This idea of going to full four show is never been sure because Supergirl was different multiverse from other three series.
The TV GUIDE stated that Medusa's storyline is a fresh air to breath compared last week's "The Darkest Places" that seems to introduce new developments. Let's not think about one part of four show crossover because it will eventually happen technically. The fact that Supergirl never exists with Flash, Green Arrow or any other legends world.
It is perfectly explainable why Supergirl is involved in three heroes episodes that will be aired this week. There is more to recommend about this crossover, however, the important part of this crossover also features a lot of familiar faces that comes from five seasons of "Arrow".
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