The CW crossover event, dubbed Heroes v Aliens, has officially kicked off in this week's Supergirl — though the connection to the wider plot arc is a lot more subtle than we expected. Season 2 Episode 8, "Medusa", acts as a nice mid-season finale for Supergirl, wrapping up plot threads while planting more hints for the second half of the season, and tying neatly in to the Heroes v Aliens plot as Kara discovers the secret behind Project Medusa.
Instead of being connected to DC's character Medusa — the actual Greek gorgon monster — this project turns out to be a Kryptonian defense measure, to be used in the event of an alien invasion. (Why the Kryptonians, an alien culture with no connection to Earth, used a figure from Greek mythology as the code for this project, remains to be seen and let's face it, will probably never be addressed. But I digress.)
Lillian Luthor attempts to hijack the Medusa virus and use it for Cadmus, effectively wiping out all aliens on Earth in one fell swoop. This was a nice demonstration of how the xenophobic nature of the Kryptonian weapon could be used for evil, forcing Kara to re-examine her home (dead) planet, as she becomes more and more disillusioned with her parents' mission to protect it.
But the Project Medusa plot isn't just another interesting demonstration of Season 2's themes — prejudice, war, immigration, and overcoming oppression — it also sets up the perfect solution to the conflict that the crossover revolves around.
Beating Back The Dominators
While his bio-weapon would be considered by many (Kara included) to be needlessly vicious and xenophobic — what about the non-Kryptonian aliens living on Krypton?! — Kara's father may have given his daughter the very tool she needed to beat back an alien invasion of her own. Or rather, of Barry's own.
As Kara deals with the problems on her Earth, the Flash's Earth (Earth-1? Earth Prime?) has been overrun with Dominators, an invading alien force intent on destroying life on Earth because... reasons. This will no doubt be the plot of the next episode in the Heroes v Aliens event, explaining why Barry and Cisco crossed dimensions to pick up some much-needed reinforcements.
After two episodes of battling the Dominators, it's possible (and even likely) that Kara will suddenly remember that she has the perfect solution waiting for her on her Earth — the formula to the Medusa virus, which handily wipes out alien life without harming humans or Kryptonians.
Of course, using the weapon in a positive way might undermine the message in "Medusa" that this virus was xenophobic and dangerous but hey, it's a tool like any other, and it seems to solve the predicament The CW heroes find themselves it.
But is this too simple a solution? Project Medusa, after being set up in the Supergirl episode, will just be sitting there like Chekov's Gun, waiting to be utilized by The CW heroes. If it isn't used then Kara is unbelievably shortsighted, and if it is then the solution to the Dominator invasion is almost too obvious. Having said that, Medusa was a neat way to tie the Supergirl episode into the crossover, despite the fact that most of the plot dealt with the arcs from Supergirl Season 2, not the Heroes v Aliens story.
There is another possibility: The Medusa virus will be suggested as a solution, but there will be some problem with using it. Maybe it poisons metahumans as well as aliens, or the physiology of the Dominators resists this virus. All that remains to be seen, but we may just have found the Super Friends' secret weapon — before the Dominator invasion has even really begun.