The time is finally here. In the mid-season finale of Supergirl, producer Greg Berlanti and his team launch the first-ever crossover between all four shows in the Arrowverse. Tonight's episode, titled "Medusa," was the start of a story based on the 1988 DC Comics crossover series Invasion!, in which an alien race called Dominators attacked earth with the goal of wiping out superheroes who possessed a metagene. The metagene is what gives superheroes their powers. It's something within them that has been unlocked to give them powers, like the Flash's super-speed.
The previous episode of Supergirl, "The Darkest Places," ended with Cyborg Superman using Kara's blood to gain access to the Fortress of Solitude. There, he ordered Superman's Kryptonian computer to teach him about a project called "Medusa," hence the title of this week's episode.
Here's the full episode description from The CW: Eliza (guest star Helen Slater) comes to town to celebrate Thanksgiving with her daughters. Alex (Chyler Leigh) decides it’s time to come out to her mother; Kara (Melisa Benoist) is shocked when Eliza suggests Mon-El (Chris Wood) has feelings for her, and Winn (Jeremy Jordan) and James (Mehcad Brooks) consider telling Kara the truth about the Guardian. Meanwhile, CADMUS unleashes a virus that instantly kills any alien in the vicinity so Kara enlists an unexpected ally – Lena Luthor (Katie McGrath). The epic superhero crossover kicks off tonight when Barry Allen (guest star Grant Gustin) and Cisco Ramon (guest star Carlos Valdes) seek Kara’s help with an alien invasion on their Earth.
After watching the episode, click through to check out photos of "Medusa." The episode debuted at 8 p.m. ET on November 28. (Bettina Strauss/The CW)