Supergirl was established as part of the Arrowverse continuum (though still in a different universe from Arrow, Flash and Legends of Tomorrow) of DC TV series in its original CBS season. But now that the series has joined its brethren on The CW those connections have become much more readily apparent -- and are soon set to explode with the upcoming week-long 'Invasion!' crossover.
Airing on Monday nights, Supergirl will be tasked with helping set the crossover up. But how does the show plan to bridge the inter-dimensional gap between the Girl of Steel and the rest of the characters? As it turns out, that will be partially answered thanks to a new Thanksgiving-themed clip that shows how things will get started.
The crossover will find multiple heroes from each show teaming up to battle fearsome alien invaders known as the Dominators. The clip in question doesn't feature said invaders or any of the other heroes, with the main notable detail being the climactic appearance of a mysterious portal that may explain how Kara Zor-El will find her way to the Arrowverse.
Instead, the clip mainly focuses on the beginning of Kara's Thanksgiving dinner, where potential important story reveals for various members of the Supergirl supporting cast looms over the proceedings (largely to Kara's bewilderment) before being interrupted by the portal. Specifically, James Olsen attempts to butter his host up with compliments seemingly in advance of revealing himself as The Guardian -- information that a visibly intoxicated Alex looks ready to reveal herself... once she also works up the courage to finally come out of the closet to the rest of her family and friends. The portal, of course, seems to put an abrupt halt to both prospective announcements.
Little else is known about what the rest of the crossover's plotline. 'Medusa' was first mentioned by the newly revealed villain Cyborg Superman at the conclusion of last week's Supergirl, where he used a stolen sample of Kara's blood to access the Fortress of Solitude and demand answers about the mysterious phrase from Superman's historical archive. What, if anything, this has to do with the Dominators is yet to be determined -- up to and including if they are a threat to both universes at once.
It has been suggested that the scene depicted in the clip is from the end of 'Medusa', and that the episode may not otherwise tie directly to Kara's eventual trip into the rest of the Arrowverse on account of the dimensional difference. It is also known that subsequent installments will tie more directly into Flash's "Flashpoint" storyline and feature an alternate-reality version of Arrow's continuity.
Source: The CW