Coming off of a food filled holiday we are reminded of our gluttonous endeavors as Supergirl and her close family and friends join together for Thanksgiving. But I ask you, what is Thanksgiving without some family arguments and hidden motives? James, Winn, and Alex argue over who gets to have their big moment and reveal a secret to the super crew (James and Winn working as team Guardian, or Alex revealing to her mom she's gay). While all the seemingly important, yet childish bickering is going on, here comes basic boy #2...I mean Mon-El coming in to sweet-talk Kara's mom in order to win brownie points and impress Kara (I see what you are doing here CW). All of this sweet madness is going on around the dinner table until what seems to be a time warp opens up in front of them (The Flash?), but only for a moment.
Flash forward a bit to the main plot of the episode. There is an airborne biochemical virus called Medusa that has slain most of the aliens residing in our favorite alien dive bar. Who else likes the slaughter of alien-kind in order to preserve the future of mankind? No one else but Cadmus's scheming behind. This Medusa virus (made of Kryptonian DNA) not only threatens the aliens of National City (except Kryptonians), but all those residing on earth. And to make matters worse, Mon-El has been infected. Luckily (in a way), the only way to fully complete this virus is through the use of a particular isotope found in Lena Luther's Luther Corp (I love the alliteration). Here comes the hard part, especially for Supergirl. She has to confront Lena about her mother as the leader of Cadmus...let's just say that conversation could've gone better.
So now we are in the thick of it. Our heroes clambering to find some sort of cure for this virus (mainly Dr. Danvers cause you know, that science degree), all the while there is tons of relationship drama and forging happening, and the fearless leader of the DEO is still shaken and having troubles with his Martian DNA. However, like the selfless leader he is he puts the needs of others before his own. The whole "Greater Good" ideal. And right on cue, the villain's grand scheme comes into fruition. With a televised signal alerting everyone in National City of the aliens impending doom, it's up to Supergirl and Martian Manhunter to do what they do best, protect the world and kick ass.
The episodes final showdown happens at the National City water port. It's Supergirl, Martian Manhunter, Cyborg Superman and the Luthers (yes both of them :'(Lena)': ). The rocket containing the Virus gets launched and Supergirl goes immediately after it. Our boy J'onn is going toe to toe with Cyborg Superman (Will the real Hank Henshaw please stand up!?) and is seemingly getting hemmed up until he goes full out beast mode and shows his (current) true form as a gigantic green/white Martian. He pretty much devastates Cryborg Superman after that. As for the rocket missile containing the Medusa virus...Supergirl was too late. As she grabbed a hold of the missile to stop it, Lillian Luther pressed that dreadful red button and exploded it knocking Supergirl back but releasing the virus. Or so we thought. As the particles and dust come crashing down containing the Medusa virus, it is shown that every alien in national city is preparing to accept his fate. Fortunately the virus was rendered useless. You want to know why? BECAUSE OUR GIRL LENA LUTHER CAME IN CLUTCH!!!!!! She switched the isotope last minute in order to fool her mother and even arrested Lillian (I guess there is at least 1 Luther you can trust). Everyone was in shock, so much so they took their eyes off of Cyborg Superman and he escaped; and that's okay. Because what was looking like to be the biggest L in Supergirl history turned out alright.
At the end of the episode, it seems like the super crew is rejoicing in a much needed W. Alex finally kisses the girl, Kara and Mon-El got a little something going, Mama Danvers is happy and she even reverse-engineered a cure to J'onn's White Martian DNA problem (SCIENCE!). Everything seems well, until the scene cuts to an alien spaceship on a voyage to earth and the alien captain of that ship makes a clear declaration that their crew is after Mon-EL (I swear basic boy #2 got issues). Well regardless, the super crew could handle that when the time comes. In that moment they needed the win. Shift to the final scene of the episode. We (the viewers) are treated to see another time/universe warp open up and both Cisco and Barry emerge from it. Barry, sadly being the bearer of bad news needs help.