“Medusa” is episode eight of season two of Supergirl and the mid-season finale.
In the previous episode, “The Darkest Place”, the reason why Cadmus wanted Kara, and for her to be human for a time, was to get some of her blood. Hank Henshaw – the real one, Cyborg Superman – headed to Superman’s Fortress of Solitude and used the blood to convince the guardian robot that he was Kara. Perhaps someone should have programmed the robot a little more carefully – the DNA might match but he and Kara look nothing alike. This was to gain knowledge of something called Medusa. The other Hank – J’onn J’onzz – discovered that the reason why Megan didn’t want him to have any of her blood in “Changing” was because she isn’t a Green Martian – but a White Martian. And giving Hank her blood will turn him into a White Martian too.
The episode opens with Kara cooking a turkey with her eyes as Eliza watches – it’s Thanksgiving. James and Winn are talking; they want to tell Kara that he is the Guardian. Only each one of them wants to do it. Alex interrupts and tells them no. She has something important to say and they are not hijacking things. Kara comes over and they dissemble, when the door goes. Mon-El has arrived and he’s brought stuffing. From his mattress. Not the intended type of stuffing. He speaks to Eliza and Kara tells Alex she thinks Mon-El is chatting Eliza up. Eliza doesn’t; she says that Mon-El is chatting her up because she’s Kara’s mother. Mon-El likes Kara. Before they eat, it is a tradition to say what they are thankful for. James starts, but Alex stops him. Before she can actually get her words out, a swirly thing opens above the table. A familiar-looking swirly thing. When it disappears, Mon-El asks if that normally happens at Thanksgiving.
At the DEO, Winn tells Kara and Alex that last night the space-time continuum ripped open in Kara’s apartment. Which was only slightly less intense than the events of the previous Thanksgiving (“Livewire”). They don’t know what caused it. Hank wants to know why Cadmus wanted Kara’s blood. Winn suggests that he hack Lena to find out what she knows. Kara says she will do it herself, all sneaky-like as a reporter. Hank agrees; Alex and Winn do not seem convinced about Kara’s ability to be sneaky. Alex tells Winn to hack Lena anyway – he was going to.
Kara tells Lena she is doing a puff piece on women in power and their mothers. Lena – who is adopted – says that she and Lillian Luthor never agreed on anything. Her mother couldn’t care less about L-Corp. Or possibly that Lena has changed the company’s direction. Afterwards, Lena calls her mother and says that they need to talk.
There’s a figure in a hood at the alien bar and it seems to leave something under it. Mon-El is there, and he sees the figure leave and follows. There’s a flashing thing under the bar’s surface. Outside, Mon-El thinks the figure is J’onn but it’s actually Cyborg Superman. Inside the bar, the flashing thing starts emitting fumes and the aliens start choking and collapsing. Mon-El came off second best in the fight and sees people fleeing. He goes inside to find out what happened and is told all the others are dead. Including an alien girl who had been chatting him up.
At the DEO, Mon-El is in quarantine until they can find out what he has been exposed to. Hank, Kara and Alex are on the other side of the door. Mon-El thinks he could have stopped it if he was inside instead of fighting Original Hank. Kara tells him he’s only alive because he was outside. The weapon killed everything that wasn’t human, so Alex suggests her mother be brought in. Hank says to Kara that he and Kara are staying in the DEO for now.
Lillian Luthor visits Lena. She thought Lena had called to make amends. During the conversation she admits she loved Lex more. Lena warns Lillian that there is a reporter sniffing around, and wants to know what she’s up to. Kara and Mon-El are playing Monopoly – with a door between them – when Kara asks Mon-El if he like likes her. Mon-El seems uncomfortable about the question – and he may be. But when he starts choking, it’s not discomfort over the question doing it.
Eliza tells them that Mon-El is stable, but infected. To catch the illness you have to come in contact with it. Eliza has analysed the virus, and she believes it’s Kryptonian in origin. Kara realises that’s what Cadmus needed her blood for, and flies out to the Fortress of Solitude. There, Kal Ex accuses her of being an intruder and tries to destroy her. She takes him out. When she looks into Project Medusa, a hologram of her father explains that it’s a weaponised virus he helped create to defend Krypton. It attacks non-Kryptonian physiology (so why is it ignoring humans?).
Kara tells the others that Cadmus has stolen the formula. Because Daxamite physiology is similar to Kryptonians, Mon-El is being affected slower, but it will kill him eventually. Kara is feeling rather disillusioned with her father and mother and talks to Hank about it. Who partially transforms into a White Martian during the conversation, so explains what happened to him.
Eliza and Alex tell them that the virus needs a dispersion agent, and L-Corp has an isotope that will do the trick. Kara heads there and Alex calls Maggie. Original Hank is entering the building and sets of the metal detector as he does. As large parts of him are now metal.
So, most of the episode is about Medusa but the swirly thing appears a couple of times. And the reason it looks familiar is because it is. It’s a breach that Cisco has opened to Kara’s universe. The story continues in The Flash “Invasion!” for this is the first in a four episode crossover between all the Arrowverse series.